SOI substrate and method for manufacturing the same

ABSTRACT

To provide an SOI substrate having a high mechanical strength, and a method for manufacturing the SOI substrate, a single crystal semiconductor substrate is irradiated with accelerated ions so that an embrittled region is formed in a region at a predetermined depth from a surface of the single crystal semiconductor substrate; the single crystal semiconductor substrate is bonded to a base substrate with an insulating layer interposed therebetween; the single crystal semiconductor substrate is heated to be separated along the embrittled region, so that a semiconductor layer is provided over the base substrate with the insulating layer interposed therebetween; and a surface of the semiconductor layer is irradiated with a laser beam so that at least a superficial part of the semiconductor layer is melted, whereby at least one of nitrogen, oxygen, and carbon is solid-dissolved in the semiconductor layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a substrate in which a single crystalsemiconductor layer is provided on an insulating layer, and a method formanufacturing the substrate. In particular, the present inventionrelates to an SOI (silicon on insulator) substrate and a method formanufacturing the same. Furthermore, the present invention relates to asemiconductor device formed using an SOI substrate and a method formanufacturing the same.

2. Description of the Related Art

In recent years, devices such as semiconductor integrated circuits,which use an SOI substrate including a thin single crystal semiconductorlayer formed on an insulating surface, have been developed instead ofthose using a bulk silicon wafer. Since the parasitic capacitancebetween a drain of a transistor and a substrate is reduced by using anSOI substrate, SOI substrates have attracted attention as substrates forimproving the performance of devices.

One of the known methods for manufacturing an SOI substrate is Smart Cut(registered trademark) method (see Patent Document 1, for example). Anoutline of the method for manufacturing an SOI substrate by Smart Cut(registered trademark) method is described below. First, hydrogen ionsare implanted into a silicon wafer by an ion implantation method so thatan embrittled region including a defect is formed at a predetermineddepth from a surface. Then, the silicon wafer into which hydrogen ionshave been implanted is bonded to another silicon wafer with a siliconoxide film interposed therebetween. After that, heat treatment isperformed so that a part of the silicon wafer into which hydrogen ionshave been implanted is separated in a thin film shape along theembrittled region. Thus, a single crystal silicon film can be obtainedover the other bonded silicon wafer. Smart Cut (registered trademark)method is also referred to as a hydrogen ion implantation separationmethod.

A method has also been proposed in which a single crystal silicon layeris formed over a base substrate made of glass by such Smart Cut(registered trademark) method (see Patent Document 2, for example).Glass substrates can have a larger area and are less expensive thansilicon wafers, and thus are mainly used for manufacturing liquidcrystal display devices and the like. By using a glass substrate as abase substrate, a large, inexpensive SOI substrate can be manufactured.

[Reference]

[Patent Document]

-   [Patent Document 1] Japanese Published Patent Application No.    H05-211128-   [Patent Document 2] Japanese Published Patent Application No.    2005-252244

SUMMARY OF THE INVENTION

In the case where a large-sized device such as a display or a devicehaving a curvature is formed using, for example, a single crystalsilicon layer formed on an insulating substrate such as a glasssubstrate as disclosed in Patent Document 2, a poor mechanical strengthof the single crystal silicon layer due to its reduced thickness hasbeen a problem. The poor mechanical strength of the single crystalsilicon layer with a reduced thickness might cause cracks and breakingof the single crystal silicon layer in the manufacture or use of thedevice. In addition, there has been a problem in that linear defects(dislocations) or planar defects occur or grow, and even when atransistor is formed using the single crystal silicon layer,characteristics of the transistor are deteriorated.

In view of the foregoing problem, one object of the present invention isto provide an SOI substrate having a high mechanical strength.

It is generally said that mechanical strength increases as the impurityconcentration in bulk silicon, specifically, the concentration ofnitrogen, oxygen, carbon, or the like in bulk silicon increases. As bulksilicon widely used for industries, for example, there is bulk siliconmanufactured by a Czochralski (CZ) method or a floating zone (FZ)method.

A single crystal semiconductor ingot or a single crystal semiconductorwafer manufactured by the CZ method contains oxygen at a concentrationof 1×10¹⁷ atoms/cm³ to 1×10¹⁸ atoms/cm³, and the concentrations ofnitrogen and carbon are 4×10¹⁵ atoms/cm³ or less and 2×10¹⁶ atoms/cm³ orless, respectively, even though they are added intentionally. Theimpurity concentration in a single crystal semiconductor ingotmanufactured by the FZ method is generally lower than that in a singlecrystal semiconductor ingot manufactured by the CZ method; therefore,the single crystal semiconductor ingot manufactured by the FZ method hasa problem of a poor mechanical strength to be easily broken. On theother hand, since the impurity concentration in bulk siliconmanufactured by the CZ method is higher than that in bulk siliconmanufactured by the FZ method, bulk silicon manufactured by the CZmethod has a higher mechanical strength.

However, when an ingot is manufactured by the CZ method, a highconcentration of nitrogen, oxygen, carbon, or the like might becontained in the ingot during a manufacturing process because of a longmelting step and a long cooling step with pulling up the ingot. Then,nitrogen, oxygen, carbon, or the like is precipitated to produce a hugedefect, which reduces the mechanical strength of the ingot. Accordingly,in order to obtain an SOI substrate having a high mechanical strength,it is preferable that nitrogen, oxygen, carbon, or the like be notcontained at a high concentration during the manufacturing of a singlecrystal silicon ingot, and the concentration of nitrogen or the like insingle crystal silicon be controlled to be within a suitable range afterlong-time melting heat treatment.

In view of the foregoing problem, another object of the presentinvention is to provide a method for manufacturing an SOI substrate witha high mechanical strength.

In addition, if nitrogen, oxygen, carbon, or the like is added duringthe growing of ingot, the concentration of nitrogen, oxygen, or the likeis determined in each ingot. Therefore, even when devices need to havedifferent performances, it is not easy to manufacture a wide variety ofdevices in small quantities. Furthermore, if oxygen or the like is addedduring the growing of ingot, a concentration gradient of oxygen or thelike exists in the direction of pulling up the ingot. Moreover, sincethe ingot is held in a quartz crucible, the concentration distributionof oxygen or the like varies between the center of the ingot and theperiphery thereof, leading to a problem of poor reproducibility of adesired concentration.

In view of the foregoing problem, still another object of the presentinvention is to provide a method for manufacturing an SOI substrate,which can be used for manufacturing a wide variety of devices in smallquantities.

One embodiment of the invention disclosed in this specification is astructure including a base substrate, an insulating layer over the basesubstrate, and a single crystal semiconductor layer over the insulatinglayer. A single crystal semiconductor substrate includes an embrittledregion formed by the addition of hydrogen, and the single crystalsemiconductor layer is obtained by separating the single crystalsemiconductor substrate along the embrittled region. The concentrationof nitrogen, oxygen, or carbon in the single crystal semiconductor layeris higher than that in a single crystal semiconductor ingot manufacturedby the CZ method or in the single crystal semiconductor substrate.

The concentration of nitrogen in the region that contains the leastamount of nitrogen in the depth direction of the single crystalsemiconductor layer is 5×10¹⁵ atoms/cm³ or more, preferably 1×10¹⁶atoms/cm³ or more, and more preferably 1×10¹⁷ atoms/cm³ or more when itis measured by SIMS (Secondary Ion Mass Spectroscopy).

The concentration of nitrogen in the single crystal semiconductor layermay be 5×10¹⁵ atoms/cm³ to 5×10¹⁹ atoms/cm³, preferably 1×10¹⁶ atoms/cm³to 2×10¹⁹ atoms/cm³, and more preferably 1×10¹⁷ atoms/cm³ to 5×10¹⁸atoms/cm³.

The single crystal semiconductor layer may include a region having anitrogen concentration of 1×10¹⁶ atoms/cm³ or more, preferably 1×10¹⁷atoms/cm³ or more, and more preferably 1×10¹⁸ atoms/cm³ or more.

The concentration of oxygen in the region that contains the least amountof oxygen in the depth direction of the single crystal semiconductorlayer is 2×10¹⁸ atoms/cm³ or more, preferably 3×10¹⁸ atoms/cm³ or more,and more preferably 5×10¹⁸ atoms/cm³ or more when it is measured bySIMS.

The concentration of oxygen in the single crystal semiconductor layermay be 2×10¹⁸ atoms/cm³ to 1×10²⁰ atoms/cm³, and preferably 3×10¹⁸atoms/cm³ to 1×10¹⁹ atoms/cm³.

The single crystal semiconductor layer may include a region having anoxygen concentration of 2×10¹⁸ atoms/cm³ or more, and preferably 5×10¹⁸atoms/cm³ or more. In the case where the concentration of oxygen iswithin the above ranges, the yield stress of the single crystalsemiconductor layer can be increased to improve the mechanical strength.

The concentration of carbon in the region that contains the least amountof carbon in the depth direction of the single crystal semiconductorlayer is 1×10¹⁷ atoms/cm³ or more, and preferably 5×10¹⁷ atoms/cm³ ormore when it is measured by SIMS.

The concentration of carbon in the single crystal semiconductor layermay be 1×10¹⁷ atoms/cm³ to 5×10²⁰ atoms/cm³, and preferably 5×10¹⁷atoms/cm³ to 5×10¹⁹ atoms/cm³.

If a region where all the concentrations of nitrogen, oxygen, and carbonare low exists in the depth direction of a single crystal semiconductorlayer with a reduced thickness, the yield stress of the region becomesextremely low to reduce the mechanical strength of the whole singlecrystal semiconductor layer. Thus, at least one of nitrogen, oxygen, andcarbon is made to have a concentration equal to or more than theaforementioned minimum concentration to increase the yield stress of thesingle crystal semiconductor layer.

When the single crystal semiconductor layer includes a region having anoxygen concentration of 2×10¹⁸ atoms/cm³ or more, and preferably 5×10¹⁸atoms/cm³ or more, the yield stress of the single crystal semiconductorlayer can be increased.

When the single crystal semiconductor layer includes a region having anitrogen concentration of 1×10¹⁶ atoms/cm³ or more, preferably 1×10¹⁷atoms/cm³ or more, and more preferably 1×10¹⁸ atoms/cm³ or more, theyield stress of the single crystal semiconductor layer can be increased.

According to another embodiment of the invention disclosed in thisspecification, a single crystal semiconductor substrate is irradiatedwith accelerated ions to add hydrogen, so that an embrittled region isformed in a region at a predetermined depth from a surface of the singlecrystal semiconductor substrate; the single crystal semiconductorsubstrate is bonded to a base substrate with an insulating layerinterposed therebetween; the single crystal semiconductor substrate isheated to be separated along the embrittled region, so that a singlecrystal semiconductor layer is provided over the base substrate with theinsulating layer interposed therebetween; and a surface of the singlecrystal semiconductor layer is irradiated with a laser beam so that atleast a superficial part of the single crystal semiconductor layer ismelted, whereby the single crystal semiconductor layer isre-single-crystallized. When the laser beam is emitted, at least oneelement of nitrogen, oxygen, and carbon is added to the single crystalsemiconductor layer. The concentration of nitrogen added to the singlecrystal semiconductor layer by laser light irradiation is 5×10¹⁵atoms/cm³ to 5×10¹⁹ atoms/cm³. The concentration of oxygen added to thesingle crystal semiconductor layer is 2×10¹⁸ atoms/cm³ to 1×10²⁰atoms/cm³. The concentration of carbon added to the single crystalsemiconductor layer is 1×10¹⁷ atoms cm³ to 5×10²⁰ atoms/cm³.

In order that at least one of nitrogen, oxygen, and carbon issolid-dissolved in or added to the single crystal semiconductor layer,the single crystal semiconductor layer is preferably irradiated with alaser beam in an atmosphere containing at least one element of nitrogen,oxygen, and carbon. Alternatively, the single crystal semiconductorlayer is preferably irradiated with a laser beam while being positivelysprayed with a gas containing at least one element of nitrogen, oxygen,and carbon.

Further alternatively, an insulating layer containing at least one ofnitrogen, oxygen, and carbon may be formed over the single crystalsemiconductor layer and the insulating layer may be irradiated with alaser beam, so that nitrogen or the like contained in the insulatinglayer is diffused into the single crystal semiconductor layer when asurface of the single crystal semiconductor layer is melted. When thesingle crystal semiconductor layer is partly melted by laser lightirradiation, nitrogen or the like can be solid-dissolved or added at alevel equal to or more than the solid solubility of nitrogen or the likein a solid-phase single crystal semiconductor through are-single-crystallizing step with a non-equilibrium state in which thesolid-phase single crystal semiconductor is melted and cooled in a shorttime. Accordingly, nitrogen or the like can be added in a shorter timeas compared to a manufacturing process of an ingot or the like requiringa long melting step. As a result, generation or growing of defects canbe prevented.

The surface of the single crystal semiconductor layer may be melted bystrong light irradiation with a lamp or the like or by irradiation withelectromagnetic waves as well as laser light irradiation.

According to another embodiment of the invention disclosed in thisspecification, hydrogen is added to a single crystal semiconductorsubstrate so that an embrittled region is formed in a region at apredetermined depth from a surface of the single crystal semiconductorsubstrate, and at the same time, at least one element of nitrogen,oxygen and carbon is added to the single crystal semiconductorsubstrate. Then, the single crystal semiconductor substrate is bonded toa base substrate with an insulating layer interposed therebetween, andthe single crystal semiconductor substrate is heated to be separatedalong the embrittled region, so that a single crystal semiconductorlayer is provided over the base substrate with the insulating layerinterposed therebetween. At the same time as the addition of hydrogen,at least one of nitrogen, oxygen, and carbon is added to the singlecrystal semiconductor substrate. Alternatively, hydrogen is added in anatmosphere containing at least one of nitrogen, oxygen, and carbon sothat the embrittled region is formed in a region at a predetermineddepth of the single crystal semiconductor substrate and at least the oneelement of nitrogen, oxygen, and carbon is added to a region from thesurface of the single crystal semiconductor substrate to the embrittledregion. As a result, the concentration of nitrogen or the like in thesingle crystal semiconductor layer that is separated later can beefficiently increased without increasing the concentration of nitrogenor the like in the single crystal semiconductor substrate before theseparation.

In this specification, a “single crystal” refers to a crystal in which,when a certain crystal axis is focused, the direction of the crystalaxis is oriented in the same direction in any portion of a sample andwhich has no crystal grain boundaries. Note that in this specification,the single crystal includes a crystal in which the direction of thecrystal axis is uniform and which has no grain boundaries as describedabove even though it includes a crystal defect or a dangling bond. Inaddition, re-single-crystallization of a single crystal semiconductorlayer means that a semiconductor layer having a single crystal structurereturns to a single crystal structure after being in a different statefrom the single crystal structure (e.g., a liquid-phase state).Alternatively, it can be said that re-single-crystallization of a singlecrystal semiconductor layer means that a single crystal semiconductorlayer is recrystallized to form a single crystal semiconductor layer.

In this specification, the concentrations of nitrogen, oxygen, andcarbon in a single crystal semiconductor layer are represented by valuesobtained by SIMS. Note that data near the interface is also picked up bythe SIMS in principle. Accordingly, the concentrations of nitrogen,oxygen, and carbon near the interface of the single crystalsemiconductor layer are detected to be higher than practicalconcentrations of the single crystal semiconductor layer. For example,in the case where a single crystal semiconductor layer is in contactwith a silicon oxide film, a silicon nitride oxide film, a siliconoxynitride film, or a silicon nitride film, the concentration of oxygenor nitrogen is higher near the interface between the single crystalsemiconductor layer and such a film. Therefore, depending on thedetection capability of SIMS, it is better not to consider theconcentration near the top surface and the bottom surface of a singlecrystal semiconductor layer, namely, the concentration of a region ofabout 10 nm (preferably 20 nm, and more preferably 25 nm) from theinterface.

Also in this specification, the minimum concentration or the lower limitof the concentration in a single crystal semiconductor layer refers tothe concentration in a region having the lowest concentration when thesingle crystal semiconductor layer is measured by SIMS in the thicknessdirection.

Note that a semiconductor device in this specification indicates all thedevices that can operate by using semiconductor characteristics, and anelectro-optical device, a semiconductor circuit, and an electronicappliance are all included in the semiconductor devices.

Note that a display device in this specification includes alight-emitting device and a liquid crystal display device in itscategory. A light-emitting device has a light-emitting element, and aliquid crystal display device has a liquid crystal element. ALight-emitting element includes, in its category, an element whoseluminance is controlled by current or voltage, and specifically includesan inorganic electroluminescent (EL) element, an organic EL element, andthe like.

When at least one element of nitrogen, oxygen, and carbon is containedin a single crystal semiconductor layer formed on an insulating surfaceat a predetermined concentration, the mechanical strength of an SOIsubstrate having the single crystal semiconductor layer can be improved.Furthermore, generation of voids in the single crystal semiconductorlayer can be suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1E are diagrams illustrating an example of a method formanufacturing an SOI substrate;

FIGS. 2A to 2D are diagrams illustrating an example of a method formanufacturing an SOI substrate;

FIG. 3 is a diagram illustrating an example of a laser irradiationapparatus used in a method for manufacturing an SOI substrate;

FIGS. 4A to 4D are diagrams illustrating an example of a method formanufacturing a semiconductor device using an SOI substrate;

FIGS. 5A to 5C are diagrams illustrating an example of a method formanufacturing a semiconductor device using an SOI substrate;

FIG. 6 is a diagram illustrating an example of a semiconductor deviceusing an SOI substrate;

FIG. 7 is a diagram illustrating an example of a semiconductor deviceusing an SOI substrate;

FIGS. 8A and 8B are diagrams illustrating an example of a display deviceusing an SOI substrate;

FIGS. 9A and 9B are diagrams illustrating an example of a display deviceusing an SOI substrate;

FIGS. 10A to 10C are views illustrating an electronic device using anSOI substrate;

FIGS. 11A and 11B are graphs each showing an example of the experimentresult of the element concentration in an SOI substrate with and withoutlaser irradiation;

FIG. 12 is a graph showing an example of the experiment result of theelement concentration in an SOI substrate with and without laserirradiation; and

FIGS. 13A and 13B are graphs each showing an example of the experimentresult of the element concentration in an SOI substrate doped withhydrogen.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the disclosed invention will be described with referenceto drawings. Note that the present invention can be implemented in awide variety of modes, and it is apparent to those skilled in the artthat modes and details can be modified without departing from the spiritand scope of the present invention. Accordingly, the present inventionshould not be construed as being limited to the description of theembodiments given below. Note that in all the drawings for explainingthe embodiments, like portions or portions having a similar function aredenoted by like reference numerals, and the description thereof isomitted.

(Embodiment 1)

In this embodiment, an example of a method for manufacturing an SOIsubstrate will be described with reference to drawings. Specifically,description will be made on the steps of forming a single crystalsemiconductor layer over a base substrate with an insulating layerinterposed therebetween by Smart Cut (registered trademark) method andrecovering the crystallinity of the single crystal semiconductor layer.

First, a single crystal semiconductor substrate 100 and a base substrate120 are prepared (see FIGS. 1A and 1B).

As the single crystal semiconductor substrate 100, for example, a singlecrystal semiconductor substrate formed of an element belonging to Group14, such as a single crystal silicon substrate, a single crystalgermanium substrate, or a single crystal silicon germanium substrate,can be used. A commercially-available silicon substrate is typically acircular substrate having a size of 5 inches (125 mm) in diameter, 6inches (150 mm) in diameter, 8 inches (200 mm) in diameter, 12 inches(300 mm) in diameter, and 16 inches (400 mm) in diameter. Note that theshape of the single crystal semiconductor substrate 100 is not limitedto a circular shape, and a single crystal semiconductor substrateprocessed into, for example, a rectangular shape can also be used. Thesingle crystal semiconductor substrate 100 is preferably manufactured bya CZ method, even though it can also be manufactured by an FZ method.

As the base substrate 120, a substrate formed of an insulator can beused. Specifically, it is possible to use a variety of glass substratesused for the electronics industry, such as an aluminosilicate glasssubstrate, an aluminoborosilicate glass substrate, or a bariumborosilicate glass substrate as well as a quartz substrate, a ceramicsubstrate, a sapphire substrate, or the like. Alternatively, a singlecrystal semiconductor substrate (for example, a single crystal siliconsubstrate) may be used as the base substrate 120. In this embodiment, acase of using a glass substrate is described. By using a glass substratewhich can have a large area and is inexpensive as the base substrate120, cost reduction can be achieved.

Next, an embrittled region 104 having a damaged crystal structure isformed at a predetermined depth from a surface of the single crystalsemiconductor substrate 100. After that, the single crystalsemiconductor substrate 100 and the base substrate 120 are bonded toeach other with an insulating layer 102 interposed therebetween (seeFIG. 1C).

The embrittled region 104 can be formed by irradiating the singlecrystal semiconductor substrate 100 with ions of hydrogen or the like.

The insulating layer 102 can be formed with a single layer or plurallayers of a silicon oxide film, a silicon oxynitride film, a siliconnitride film, and/or a silicon nitride oxide film. These films can beformed by thermal oxidation, CVD, sputtering, or the like.

In this specification, a silicon oxynitride film refers to a film thatcontains more oxygen than nitrogen and, preferably, in the case wheremeasurements are performed using Rutherford backscattering spectrometry(RBS) and hydrogen forward scattering (HFS), includes oxygen, nitrogen,silicon, and hydrogen at concentrations ranging from 50 at. % to 70 at.%, 0.5 at. % to 15 at. %, 25 at. % to 35 at. %, and 0.1 at. % to 10 at.%, respectively. On the other hand, a silicon nitride oxide film refersto a film that contains more nitrogen than oxygen and, preferably, inthe case where measurements are performed using RBS and HFS, includesoxygen, nitrogen, silicon, and hydrogen at concentrations ranging from 5at. % to 30 at. %, 20 at. % to 55 at. %, 25 at. % to 35 at. %, and 10at. % to 30 at. %, respectively. Note that percentages of nitrogen,oxygen, silicon, and hydrogen fall within the ranges given above, wherethe total number of atoms contained in the silicon oxynitride film orthe silicon nitride oxide film is defined as 100 at. %.

Next, heat treatment is performed to separate the single crystalsemiconductor substrate 100 along the embrittled region 104, whereby asingle crystal semiconductor layer 124 (also simply referred to as asemiconductor layer) is provided over the base substrate 120 with theinsulating layer 102 interposed therebetween (see FIG. 1D).

When the heat treatment is performed, the element added is precipitatedinto crystal defects formed in the embrittled region 104 by temperatureincrease, and the internal pressure of the crystal defects increases.The increase in pressure changes the volume of the crystal defects inthe embrittled region 104 to generate cracks in the embrittled region104, whereby the single crystal semiconductor substrate 100 is separatedalong the embrittled region 104. Since the insulating layer 102 isbonded to the base substrate 120, the single crystal semiconductor layer124 that is separated from the single crystal semiconductor substrate100 is provided over the base substrate 120.

Next, the surface of the single crystal semiconductor layer 124 providedover the base substrate 120 is irradiated with a laser beam 130, so thatthe crystallinity of the single crystal semiconductor layer 124 isrecovered (re-single-crystallization) (see FIG. 1E). The irradiationwith the laser beam 130 is performed in an atmosphere containing atleast one of nitrogen, oxygen, and carbon. Alternatively, theirradiation with the laser beam 130 is performed while the singlecrystal semiconductor layer 124 is sprayed with a gas containing atleast one of nitrogen, oxygen, and carbon.

In general, the crystallinity of a superficial part of the singlecrystal semiconductor layer 124 provided over the base substrate 120after the separation is damaged because of crystal defects and the likeformed due to the formation of the embrittled region 104, the separationalong the embrittled region 104, and the like. Accordingly, the surfaceof the single crystal semiconductor layer 124 is irradiated with thelaser beam 130 as illustrated in FIG. 1E, whereby at least thesuperficial part of the single crystal semiconductor layer 124 is meltedto recover the crystallinity. Note that the crystallinity of the singlecrystal semiconductor layer 124 can be evaluated by observation with anoptical microscope, Raman shift and full width at half maximum which areobtained from a Raman spectroscopy spectrum, or the like.

In this embodiment, it is preferable that the single crystalsemiconductor layer 124 be melted not completely but partially (partialmelting) by emitting the laser beam 130 from the side of the surface ofthe single crystal semiconductor layer 124 exposed by the separation.The partial melting means that a portion of the single crystalsemiconductor layer 124, which is melted by the irradiation with thelaser beam 130, has a depth smaller than the distance between thesurface and the interface with the insulating layer 102 (the thicknessof the single crystal semiconductor layer 124). That is, the upper partof the single crystal semiconductor layer 124 is melted to be broughtinto a liquid phase whereas the lower part thereof is kept in a solidphase without being melted.

By the partial melting of the single crystal semiconductor layer 124,crystals at the part melted by the irradiation with the laser beam 130grow along a plane direction of the non-melted part of the singlecrystal semiconductor layer; therefore, recrystallization can beperformed with the plane direction aligned as compared with the case ofcomplete melting. At this time, since the partial melting is performedin an atmosphere containing nitrogen, oxygen, or carbon, the element inthe atmosphere is contained in the single crystal semiconductor layer124. While the superficial part of the single crystal semiconductorlayer 124 is melted, the solid solubility of nitrogen or the likeincreases so that nitrogen or the like in the atmosphere is diffusedinto the single crystal semiconductor layer 124 which is being melted.Thus, the concentration of nitrogen or the like in the single crystalsemiconductor layer 124 solidified can be made higher.

In this embodiment, the irradiation with the laser beam 130 is performedin a chamber having a nitrogen atmosphere. When the single crystalsemiconductor layer 124 is irradiated with the laser beam 130 in such anatmosphere to be melted, nitrogen in the atmosphere is easily containedin the single crystal semiconductor layer 124. The contained nitrogenatoms are trapped in the dislocation in the single crystal semiconductorlayer 124, and the dislocation is fixed. As a result, the yield stressincreases to improve the mechanical strength of the single crystalsemiconductor layer.

Furthermore, the addition of nitrogen can reduce the size or the amountof change in the size of voids generated in the single crystalsemiconductor layer 124. Note that nitrogen has a stronger effect offixing than oxygen; thus, it is preferable to add nitrogen to fix thedislocation.

On the other hand, it is effective to add oxygen in consideration of thediffusion rate and solid solubility. Therefore, the laser beam 130 maybe emitted in an oxygen atmosphere or an atmosphere containing oxygenand nitrogen instead of a nitrogen atmosphere. The addition of oxygen inthe single crystal semiconductor layer 124 also produces an effect thatthe interstitial oxygen concentration of the single crystalsemiconductor layer 124 increases and the single crystal semiconductorlayer 124 does not warp easily.

In this embodiment, carbon may be added instead of nitrogen; however, itis particularly preferable that carbon be added in addition to nitrogen.This is because carbon has an effect of reducing crystal defects due tothe addition of nitrogen, and the addition of carbon makes it possibleto easily control defects. Any of such a nitrogen gas, an oxygen gas,and a gas containing carbon as one of its components may be selected asappropriate to be introduced into a chamber in which the laserirradiation process is performed.

When a nitrogen gas or an oxygen gas is introduced using a gas refiningapparatus (purification apparatus), the purity of a supplied gas can beimproved and the concentration of nitrogen or oxygen in the atmospherecan be controlled to be effectively increased. Alternatively, theirradiation with the laser beam 130 may be performed in areduced-pressure atmosphere.

Specifically, the minimum concentration of nitrogen in the singlecrystal semiconductor layer 124 including the portion melted by theirradiation with the laser beam 130 may be 5×10¹⁵ atoms/cm³ or more,preferably 1×10¹⁶ atoms/cm³ or more, and more preferably 1×10¹⁷atoms/cm³ or more. Alternatively, the single crystal semiconductor layer124 may include a region having a nitrogen concentration of 1×10¹⁶atoms/cm³ or more, preferably 1×10¹⁷ atoms/cm³ or more, and morepreferably 1×10¹⁸ atoms/cm³ or more. In consideration of thecharacteristics of a transistor using the single crystal semiconductorlayer, the upper limit of the concentration of nitrogen is preferablydetermined: 5×10¹⁹ atoms/cm³ or less, preferably 2×10¹⁹ atoms/cm³ orless, and more preferably 5×10¹⁸ atoms/cm³ or less. Note that thenitrogen concentration in the single crystal semiconductor layer 124 canbe measured by SIMS.

Alternatively, the laser beam 130 may be emitted while the singlecrystal semiconductor layer 124 is positively sprayed with a nitrogengas (N₂ gas) instead of being in a nitrogen atmosphere. Furtheralternatively, the laser beam 130 may be emitted while the singlecrystal semiconductor layer 124 is sprayed with a nitrogen gas in anitrogen atmosphere. As a result, nitrogen elements are contained(solid-dissolved) in the single crystal semiconductor layer 124 moreeasily. It is needless to say that the single crystal semiconductorlayer 124 may be sprayed with an oxygen gas (O₂ gas) or a gas containingcarbon as one of its components (e.g., CO₂ gas) instead of a nitrogengas. Furthermore, the laser beam 130 may be emitted while the singlecrystal semiconductor layer 124 is sprayed with a gas containing atleast two or more elements of nitrogen, oxygen, and carbon. The minimumconcentration of oxygen in the single crystal semiconductor layer 124may be 2×10¹⁸ atoms/cm³ or more, preferably 3×10¹⁸ atoms/cm³ or more,and more preferably 5×10¹⁸ atoms/cm³ or more. In consideration of thecharacteristics of a transistor using the single crystal semiconductorlayer, the upper limit of the concentration of oxygen is preferably1×10²⁰ atoms/cm³ or less, and more preferably 1×10¹⁹ atoms/cm³ or less.Alternatively, the single crystal semiconductor layer 124 may include aregion having an oxygen concentration of 2×10¹⁸ atoms/cm³ or more, andpreferably 5×10¹⁸ atoms/cm³ or more to improve the mechanical strength.The minimum concentration of carbon in the single crystal semiconductorlayer 124 may be 1×10¹⁷ atoms/cm³ or more, and preferably 5×10¹⁷atoms/cm³ or more. In consideration of the characteristics of atransistor using the single crystal semiconductor layer, the upper limitof the concentration of carbon is preferably 5×10²⁰ atoms/cm³ or less,and more preferably 5×10¹⁹ atoms/cm³ or less.

As a laser which can be used in this embodiment, a laser having anoscillation wavelength in the range of the ultraviolet to visible lightregion is selected. The laser beam 130 has a wavelength which isabsorbed by the single crystal semiconductor layer 124, and thewavelength can be determined in consideration of the skin depth of thelaser beam, and the like. For example, the wavelength can be set in therange of 250 nm to 700 nm.

As the laser, a pulsed laser or a continuous wave laser (CW laser) canbe used. For example, a pulsed laser preferably has a repetition rate ofless than 10 MHz and a pulse width of 10 nanoseconds to 500 nanoseconds.A typical pulsed laser is an excimer laser that emits a laser beamhaving a wavelength of 400 nm or less. As the aforementioned excimerlaser, for example, it is possible to use a XeCl excimer laser having arepetition rate of 10 Hz to 300 Hz, a pulse width of 25 nanoseconds, anda wavelength of 308 nm. In addition, in scanning with the pulsed laserbeam, one shot and the following shot may be partially overlapped witheach other. By partially overlapping one shot with the following shot inlaser irradiation, partial refining of single crystals is performedrepeatedly, whereby a single crystal semiconductor layer havingexcellent characteristics can be obtained.

Further, the range of the energy density of the laser beam to partiallymelt the single crystal semiconductor layer 124 is set to such an energydensity that the single crystal semiconductor layer 124 is notcompletely melted, in consideration of the wavelength of the laser beam,the skin depth thereof, the thickness of the single crystalsemiconductor layer 124, and the like. For example, when the thicknessof the single crystal semiconductor layer 124 is large, the energynecessary for completely melting the single crystal semiconductor layer124 is also large, and thus the range of the energy density of the laserbeam can be set to be wide. On the other hand, when the thickness of thesingle crystal semiconductor layer 124 is small, the energy necessaryfor completely melting the single crystal semiconductor layer 124 isalso small, and thus the energy density of the laser beam is preferablyset to be small. Note that when the single crystal semiconductor layer124 is irradiated with the laser beam while being heated, the upperlimit of the range of the energy density necessary for partially meltingis preferably set to be small to prevent completely melting of thesingle crystal semiconductor layer 124.

By employing the method described in this embodiment, even when thesingle crystal semiconductor layer provided over the base substrate ismelted by irradiation with the laser beam to recover the crystallinity,at least one of nitrogen, oxygen, and carbon is contained in the singlecrystal semiconductor layer in the laser irradiation process; thus,generation of voids in the single crystal semiconductor layer can beprevented.

In addition, these atoms are trapped in the dislocation in the singlecrystal semiconductor layer to fix the dislocation, whereby themechanical strength of the single crystal semiconductor layer can beimproved.

A large amount of nitrogen or the like can be intentionally contained ina single crystal semiconductor ingot during the manufacturing process.In that case, however, such an element is precipitated to produce a hugedefect because of a long melting step and a long cooling step withpulling up the ingot. Moreover, a concentration gradient of oxygenexists in the direction of pulling up the ingot. Furthermore, since theingot is held in a quartz crucible, the concentration distribution ofoxygen or the like varies between the center of the ingot and theperiphery thereof.

According to one embodiment of the invention disclosed in thisembodiment, nitrogen or the like is solid-dissolved during partialmelting by laser irradiation, which reduces the melting time and coolingtime and prevents generation and growing of defects.

Furthermore, the concentration of nitrogen or the like can be controlledfor each single crystal semiconductor layer after the manufacturing ofthe ingot; therefore, a wide variety of SOI substrates each satisfying agiven specification can be manufactured in small quantities.

In addition, the SOI substrate manufactured in this embodiment has animproved mechanical strength, and thus is suitable to be used for aflexible device.

Although the surface of the single crystal semiconductor layer 124 isdirectly irradiated with the laser beam 130 in this embodiment, a filmcontaining at least one of nitrogen, oxygen, and carbon may be formed onthe single crystal semiconductor layer 124, and a surface of the filmmay be irradiated with the laser beam 130. As the film, for example, asilicon nitride film, a silicon oxide film, a silicon carbide film, asilicon nitride oxide film, or a silicon oxynitride film may be used.When the film is irradiated with the laser beam 130, the element such asnitrogen or oxygen in the film is contained in the single crystalsemiconductor layer 124. As a result, the concentration of nitrogen,oxygen, or carbon which is solid-dissolved in the single crystalsemiconductor layer 124 can be further increased.

When the concentration of oxygen, nitrogen, or carbon in the singlecrystal semiconductor layer 124 is higher than that in a single crystalsemiconductor ingot manufactured by the CZ method, a device including atransistor and the like using the single crystal semiconductor layer 124can have an improved mechanical strength. Accordingly, when the singlecrystal semiconductor layer provided over the base substrate, which isdescribed in this embodiment, is used as an active layer and the like ofa transistor in a display and the like, the display is not easilydamaged even when, for example, bending stress is applied to thedisplay. Furthermore, in this embodiment, an element such as nitrogen oroxygen may be added at the same time as adding hydrogen to form anembrittled region. Thus, the mechanical strength of a device obtainedcan be improved without requiring additional steps. In addition, anelement such as nitrogen or oxygen is added during a laser irradiationprocess, resulting in an improvement in the mechanical strength of adevice without requiring additional steps. Moreover, since the singlecrystal semiconductor layer is partially melted by being irradiated witha laser beam, an element such as nitrogen or oxygen is easilysolid-dissolved and the concentration of such an element can beincreased to a desired level.

Note that the structure shown in this embodiment can be combined withstructures shown in other embodiments in this specification asappropriate.

(Embodiment 2)

In this embodiment, bonding of the single crystal semiconductorsubstrate 100 to the base substrate 120 will be described in detail withreference to drawings.

First, the single crystal semiconductor substrate 100 is prepared (seeFIG. 2A-1). It is preferable that the surface of the single crystalsemiconductor substrate 100 be cleaned in advance with a sulfuricacid/hydrogen peroxide mixture (SPM), an ammonium hydroxide/hydrogenperoxide mixture (APM), a hydrochloric acid/hydrogen peroxide mixture(HPM), hydrofluoric acid/hydrogen peroxide mixture (FPM), dilutehydrofluoric acid (DHF), or the like to remove dust. Dilute hydrofluoricacid and ozone water may be discharged alternately for cleaning. In thecase where a natural oxide film is formed on the surface of the singlecrystal semiconductor substrate 100, the natural oxide film can beremoved by alternately discharging ozone water and dilute hydrofluoricacid on the surface of the single crystal semiconductor substrate 100while rotating the single crystal semiconductor substrate 100 in thehorizontal direction. After the natural oxide film is removed, achemical oxide may be formed on the single crystal semiconductorsubstrate 100, which can suppress the generation of watermarks duringcleaning and drying. The chemical oxide is not necessarily formed if nowatermark is generated.

Next, an oxide film 132 is formed on the surface of the single crystalsemiconductor substrate 100 (see FIG. 2A-2).

The oxide film 132 can be formed with a single layer or plural layers ofa silicon oxide film, a silicon oxynitride film and/or the like. Thesefilms can be formed by thermal oxidation, CVD, sputtering, or the like.In the case where the oxide film 132 is formed by CVD, a silicon oxidefilm formed using organosilane such as tetraethoxysilane (abbreviation:TEOS, chemical formula: Si(OC₂H₅)₄) is preferably used as the oxide film132 in terms of productivity.

In this embodiment, the single crystal semiconductor substrate 100 issubjected to thermal oxidation treatment to form the oxide film 132(here, SiO_(x) film) (see FIG. 2A-2). The thermal oxidation treatment ispreferably performed in an oxidizing atmosphere to which halogen isadded.

For example, the single crystal semiconductor substrate 100 is subjectedto thermal oxidation treatment in an oxidizing atmosphere to whichhydrogen chloride is added, so that the oxide film 132 is formed. Inthat case, the oxide film 132 contains a chlorine atom.

The chlorine atom contained in the oxide film 132 forms distortion. As aresult, the moisture absorption rate of the oxide film 132 increases andthe diffusion rate increases. That is, in the case where moisture ispresent on the surface of the oxide film 132, the moisture can berapidly absorbed and diffused in the oxide film 132. In addition,defects due to oxide precipitated in a single crystal semiconductor filmcan be eliminated. Furthermore, a chloride of a heavy metal (such as Fe,Cr, Ni, or Mo) which is an extrinsic impurity can be formed to performchemical gettering with outward diffusion so that the heavy metal isremoved from the single crystal semiconductor substrate 100.

For example, the thermal oxidation treatment can be performed at atemperature of 750° C. to 1150° C., and preferably 900° C. to 1100° C.(typically, 1000° C.) in an oxidizing atmosphere containing hydrogenchloride (HCl) at 0.5 vol % to 10 vol % (preferably 2 vol %) withrespect to oxygen. Treatment time may be 0.1 to 6 hours, preferably 0.5to 1 hour. The thickness of the oxide film that is formed can be 10 nmto 1000 nm (preferably 50 nm to 200 nm), for example, 100 nm. Theformation of the oxide film 132 in an oxidizing atmosphere containinghydrogen chloride can increase the withstand voltage and reduce theinterface state density between the single crystal semiconductorsubstrate 100 and the oxide film 132.

In this embodiment, the concentration of chlorine in the oxide film 132is controlled to 1×10¹⁷ atoms/cm³ to 1×10²¹ atoms/cm³.

When halogen such as chlorine is contained in the oxide film 132 by HCloxidation or the like, an impurity which adversely affects the singlecrystal semiconductor substrate (for example, a mobile ion such as Na)can be gettered. That is, through the heat treatment performed after theformation of the oxide film 132, the impurity contained in the singlecrystal semiconductor substrate is precipitated in the oxide film 132,and reacts with halogen (such as chlorine) to be captured orneutralized. Thus, the impurity captured in the oxide film 132 can befixed to prevent contamination of the single crystal semiconductorsubstrate 100. Furthermore, when the oxide film 132 is bonded to a glasssubstrate, the oxide film 132 can serve as a film which fixes animpurity such as Na contained in the glass.

In particular, halogen such as chlorine contained in the oxide film 132by HCl oxidation or the like is effective to remove contamination of asemiconductor substrate which is not cleaned sufficiently or asemiconductor substrate which is reused repeatedly.

The halogen atom to be contained in the oxide film 132 is not limited toa chlorine atom. A fluorine atom may be contained in the oxide film 132.In order that the surface of the single crystal semiconductor substrate100 is fluoro-oxidized, the surface of the single crystal semiconductorsubstrate 100 may be soaked in HF solution and then subjected to thermaloxidation in an oxidizing atmosphere, or thermal oxidation may beperformed on the single crystal semiconductor substrate 100 in anoxidizing atmosphere to which NF₃ is added.

Furthermore, after the thermal oxidation treatment is performed in anoxidizing atmosphere containing hydrogen chloride, heat treatment ispreferably performed in a nitrogen atmosphere. As a result, defects canbe reduced.

Then, the single crystal semiconductor substrate 100 is irradiated withions having kinetic energy, whereby the embrittled region 104 whosecrystal structure is damaged is formed at a predetermined depth of thesingle crystal semiconductor substrate 100 (see FIG. 2A-3). When thesingle crystal semiconductor substrate 100 is irradiated with ions 103which are accelerated through the oxide film 132 as illustrated in FIG.2A-3, the ions 103 are added to a region at a predetermined depth fromthe surface of the single crystal semiconductor substrate 100 and theembrittled region 104 can be formed. The ions 103 are obtained asfollows: a source gas is excited to generate plasma of the source gas,and ions included in this plasma are extracted from the plasma by theaction of an electric field and then accelerated.

The depth at which the embrittled region 104 is formed can be controlledby the kinetic energy, mass, charge, and incident angle of the ions 103.The kinetic energy can be controlled by an acceleration voltage, dosage,or the like. The embrittled region 104 is formed at a depthsubstantially equal to the average penetration depth of the ions 103.Therefore, the thickness of a single crystal semiconductor layerseparated from the single crystal semiconductor substrate 100 isdetermined by the implantation depth of the ions 103. The depth at whichthe embrittled region 104 is formed is controlled so that the thicknessof the single crystal semiconductor layer is 50 nm to 500 nm, andpreferably 100 nm to 200 nm.

The embrittled region 104 can be formed by ion doping treatment. The iondoping treatment can be performed using an ion doping apparatus. An iondoping apparatus is typically a non-mass-separation apparatus forirradiating a processing object disposed in a chamber with all kinds ofion species which are generated by plasma excitation of a process gas.The apparatus is called “non-mass-separation apparatus” because aprocessing object is irradiated with all kinds of ion species withoutmass separation of ion species in plasma. In contrast, anion-implantation apparatus is a mass-separation apparatus. Theion-implantation apparatus is an apparatus with which a processingobject is irradiated with ion species having a specific mass throughmass separation of ion species in plasma.

Main components of the ion doping apparatus are as follows: a chamber inwhich a processing object is disposed, an ion source for generatingdesired ions, and an acceleration mechanism for accelerating andemitting ions. The ion source includes a gas supply device for supplyinga source gas to generate a desired ion species, an electrode forexciting the source gas to generate plasma, and the like. As theelectrode for generating plasma, a filament electrode, a capacitivecoupling high-frequency discharging electrode, or the like is used. Theacceleration mechanism includes electrodes such as an extractionelectrode, an acceleration electrode, a deceleration electrode, and aground electrode; a power supply for supplying power to theseelectrodes; and the like. These electrodes that are included in theacceleration mechanism are provided with a plurality of openings orslits, through which the ions that are generated from the ion source areaccelerated. Note that the components of the ion doping apparatus arenot limited to the above-mentioned components, and another mechanism isprovided as needed.

In this embodiment, hydrogen is added to the single crystalsemiconductor substrate 100 with an ion doping apparatus. A gascontaining hydrogen, for example, H₂, is supplied as a plasma sourcegas. A hydrogen gas is introduced into a vacuum, and RF power isintroduced by a capacitive coupling method or an inductive couplingmethod, so that hydrogen molecules are excited to generate plasma. Then,ions included in plasma are accelerated without mass separation, and thesingle crystal semiconductor substrate 100 is irradiated with theaccelerated ions. Plasma may be generated in such a manner that a directcurrent flows through a high dielectric constant metal such as tungstento emit thermal electrons, and the thermal electrons collide with thehydrogen gas. A vacuum atmosphere is kept by evacuation with aturbo-molecular pump while introducing a hydrogen gas. The vacuumatmosphere may have a pressure of 1×10⁻³ Pa to 1×10⁻¹ Pa by introducinga hydrogen gas into an atmosphere having a back-pressure of 1×10⁻⁶ Pa to1×10⁻⁴ Pa.

In an ion doping apparatus, the percentage of H₃ ⁺ to the total amountof ion species (H⁺, H₂ ⁺, and H₃ ⁺) that are generated from a hydrogengas is set to 50% or more. More preferably, the percentage of H₃ ⁺ isset to 80% or more. Since mass separation is not performed in the iondoping apparatus, the percentage of one kind (H₃ ⁺) to plural kinds ofion species generated in plasma is preferably set to 50% or more, andmore preferably 80% or more. By irradiation with ions having the samemass, ions can be added in a concentrated manner to the same depth inthe single crystal semiconductor substrate 100.

In order to form the embrittled region 104 in a shallow region, theacceleration voltage for the ions 103 needs to be low. With an increasein the percentage of H₃ ⁺ ions in plasma, atomic hydrogen (H) can beefficiently added to the single crystal semiconductor substrate 100.Since the mass of H₃ ⁺ ions is three times as large as that of H⁺ ions,when one hydrogen atom is added to the same depth, the accelerationvoltage of H₃ ⁺ ions can be three times as high as that of H⁺ ions. Whenthe acceleration voltage of ions can be increased, the takt time of anion irradiation step can be shortened, resulting in an improvement inproductivity and throughput.

Ion doping apparatuses are inexpensive and excellent for use inlarge-area treatment. Therefore, by irradiation with H₃ ⁺ by use of suchan ion doping apparatus, significant effects such as an improvement insemiconductor characteristics, an increase in area, a reduction incosts, and an improvement in productivity can be obtained.

Note that the step of irradiating the single crystal semiconductorsubstrate 100 with the ions 103 which are accelerated can also beperformed by an ion implantation apparatus. An ion implantationapparatus is a mass-separation apparatus with which a processing objectwhich is disposed in a chamber is irradiated with an ion species havinga specific mass after mass separation of plural kinds of ion speciesgenerated by excitation of a source gas into plasma. Thus, when an ionimplantation apparatus is used, H⁺ ions and H₂ ⁺ ions that are generatedby excitation of a hydrogen gas or PH₃ are subjected to mass separation,and either H⁺ ions or H₂ ⁺ ions are accelerated, with which the singlecrystal semiconductor substrate 100 is irradiated.

In this embodiment, nitrogen and oxygen as well as hydrogen are added tothe single crystal semiconductor substrate 100. Carbon may also be addedin addition to nitrogen and oxygen. Alternatively, one or a combinationof nitrogen, oxygen, and carbon may be added. In the case of using anion doping apparatus, the single crystal semiconductor substrate 100 ismainly doped with H₃ ⁺ ions, and also doped with O₂ ⁺ ions or N₂ ⁺ ionsat the same time. O₂ ⁺ ions and N₂ ⁺ ions have a larger mass than H₃ ⁺ions, and thus H₃ ⁺ ions are added at a deeper depth in the singlecrystal semiconductor substrate 100. Accordingly, the embrittled region104 is formed at a depth substantially equal to the average penetrationdepth of H₃ ⁺ ions, and a region shallower than the embrittled region104, namely, a region which is separated later to be the single crystalsemiconductor layer 124 is doped with a large amount of O₂ ⁺ ions or N₂⁺ ions. As a result, the mechanical strength of the single crystalsemiconductor layer can be improved. With the ion implantation apparatusand the ion doping apparatus, an element directly added to a substratecan be selected, and thus the concentration of nitrogen or the like inthe substrate can be controlled more easily than with a laserirradiation apparatus.

The concentration of nitrogen in the region between the surface of thesingle crystal semiconductor substrate 100 and the embrittled region 104may be 5×10¹⁵ atoms/cm³ or more, preferably 1×10¹⁶ atoms/cm³ or more,and more preferably 1×10¹⁷ atoms/cm³ or more. The concentration ofoxygen may be 2×10¹⁸ atoms/cm³ or more, preferably 3×10¹⁸ atoms/cm³ ormore, and more preferably 5×10¹⁸ atoms/cm³ or more. In order to controlthe concentration of oxygen, it is also effective to provide a regionhaving an oxygen concentration of 2×10¹⁸ atoms/cm³ or more, andpreferably 5×10¹⁸ atoms/cm³ or more. In the case where carbon is addedin addition to nitrogen and oxygen, or only carbon is added, theconcentration of carbon may be 1×10¹⁷ atoms/cm³ or more, and preferably5×10¹⁷ atoms/cm³ or more. Note that each of the concentrations isrepresented by a value obtained by SIMS. When one of nitrogen, oxygen,and carbon is contained in the region between the surface of the singlecrystal semiconductor substrate 100 and the embrittled region 104 so asto be within the above range, the mechanical strength of the singlecrystal semiconductor layer 100 can be improved and growing ofdislocation can be suppressed. In particular, it is preferable to addcarbon in addition to nitrogen because carbon has an effect of reducingcrystal defects due to the addition of nitrogen. Furthermore, if theoxide film 132 is formed by oxidation, oxygen in the single crystalsemiconductor substrate 100 is extracted by high-heat treatment, leadingto a decrease in the concentration of oxygen in the substrate.Accordingly, oxygen is preferably added in addition to hydrogen in orderto increase the concentration of oxygen in the single crystalsemiconductor substrate 100, which has been decreased.

It is also possible to use nitrogen or oxygen contained in theback-pressure when plasma of a hydrogen gas is generated by introducinga hydrogen gas into a vacuum atmosphere. In the case of adding nitrogen,oxygen, or carbon in addition to hydrogen, an ion doping apparatus ispreferably used. This is because, since mass separation is not performedin the ion doping apparatus, plural kinds of ions selected fromnitrogen, oxygen, and carbon can be simultaneously added and thepercentage of ions including atoms other than hydrogen can be controlledby the ratio between the back-pressure and the processing pressure, theplasma generating method, and the like.

Next, the base substrate 120 is prepared (see FIG. 2B-1).

As the base substrate 120, a substrate formed of an insulator is used.Specifically, it is possible to use a variety of glass substrates usedfor the electronics industry, such as an aluminosilicate glasssubstrate, an aluminoborosilicate glass substrate, a barium borosilicateglass substrate, a lanthana aluminosilicate glass substrate, or atitania lanthana aluminosilicate glass substrate as well as a quartzsubstrate, a ceramic substrate, a sapphire substrate, or the like. Inthis embodiment, a case of using a glass substrate is described. Byusing a glass substrate which can have a large area and is inexpensiveas the base substrate 120, cost reduction can be achieved.

It is preferable that the surface of the base substrate 120 be cleanedbefore being used. Specifically, the base substrate 120 is subjected toultrasonic cleaning using hydrochloric acid/hydrogen peroxide mixture(HPM), sulfuric acid/hydrogen peroxide mixture (SPM), ammoniumhydroxide/hydrogen peroxide mixture (APM), dilute hydrofluoric acid(DHF), or the like. For example, the surface of the base substrate 120is preferably subjected to ultrasonic cleaning using hydrochloricacid/hydrogen peroxide mixture. Through such cleaning treatment, thesurface of the base substrate 120 can be planarized and abrasiveparticles remaining on the surface of the base substrate 120 can beremoved.

Next, a nitrogen-containing layer 121 (for example, an insulating filmcontaining nitrogen, such as a silicon nitride film (SiN_(x)) or asilicon nitride oxide film (SiN_(x)O_(y))(x>y)) is formed on the surfaceof the base substrate 120 (see FIG. 2B-2).

In this embodiment, the nitrogen-containing layer 121 serves as a layer(a bonding layer) bonded to the oxide film 132 provided on the singlecrystal semiconductor substrate 100. In addition, when a single crystalsemiconductor layer having a single crystal structure is provided overthe base substrate later, the nitrogen-containing layer 121 also servesas a barrier layer for preventing impurities such as sodium (Na)contained in the base substrate from diffusing into the single crystalsemiconductor layer.

Since the nitrogen-containing layer 121 is used as the bonding layer, itis preferable that the nitrogen-containing layer 121 have a smoothsurface in order to suppress defective bonding. Specifically, thenitrogen-containing layer 121 is formed to have a surface with anaverage surface roughness (Ra) of 0.5 nm or less and a root-mean-squaresurface roughness (Rms) of 0.60 nm or less, preferably, an averagesurface roughness of 0.20 nm or less and a root-mean-square surfaceroughness of 0.25 nm or less. The thickness of the nitrogen-containinglayer 121 is preferably in the range of 10 nm to 200 nm, and morepreferably 50 nm to 100 nm.

Next, the surface of the single crystal semiconductor substrate 100 andthe surface of the base substrate 120 are disposed to face each otherand then the surface of the oxide film 132 and the surface of thenitrogen-containing layer 121 are bonded together (see FIG. 2C).

Here, after the single crystal semiconductor substrate 100 and the basesubstrate 120 are bonded together with the oxide film 132 and thenitrogen-containing layer 121 interposed therebetween, a pressure ofapproximately 1 N/cm² to 500 N/cm², and preferably 1 N/cm² to 20 N/cm²is applied to a part of the single crystal semiconductor substrate 100.Bonding of the oxide film 132 and the nitrogen-containing layer 121begins at the part to which the pressure is applied and then extendsspontaneously throughout the surface. This bonding step, which isperformed by the action of van der Waals force or hydrogen bonding, canbe performed at room temperature without any heat treatment. Thus, asubstrate having a low temperature limit, such as a glass substrate, canbe used as the base substrate 120. In the bonding step, particularlywhen the bonding surfaces are hydrophilic and hydrophilic groups such asOH groups or water molecules (H₂O) are present on the bonding surfaces,hydrogen bonds are easily formed to promote the bonding. Such aphenomenon proceeds rapidly if the bonding surfaces are smooth, becausethe distance between the bonding surfaces is easily reduced.

Accordingly, before the single crystal semiconductor substrate 100 isbonded to the base substrate 120, surface treatment is preferablyperformed on the oxide film 132 formed over the single crystalsemiconductor substrate 100 and the nitrogen-containing layer 121 formedover the base substrate 120.

As the surface treatment, plasma treatment, ozone treatment, megasoniccleaning, two-fluid cleaning (a method for spraying functional watersuch as pure water or hydrogen-containing water with a carrier gas suchas nitrogen), or a combination thereof can be performed. Specifically,ozone treatment, megasonic cleaning, two-fluid cleaning, or the like isperformed after the surface of at least one of the oxide film 132 andthe nitrogen-containing layer 121 is subjected to plasma treatment,whereby dust such as an organic substance on the surface of the oxidefilm 132 and the nitrogen-containing layer 121 can be removed and thesurfaces can be made hydrophilic. As a result, the bonding strengthbetween the oxide film 132 and the nitrogen-containing layer 121 can beincreased.

Here, an example of ozone treatment is described. For example, ozonetreatment can be performed on a surface of a processing object byirradiation with ultraviolet (UV) light in an atmosphere containingoxygen. The ozone treatment in which irradiation with ultraviolet lightis performed in an atmosphere containing oxygen is also referred to asUV ozone treatment, ultraviolet ozone treatment, or the like. Byirradiation with ultraviolet light having a wavelength of lower than 200nm and ultraviolet light having a wavelength of 200 nm or higher in anatmosphere containing oxygen, ozone can be generated and singlet oxygencan be generated from the ozone. By irradiation with ultraviolet lighthaving a wavelength of lower than 180 nm, ozone can be generated andsinglet oxygen can also be generated from the ozone.

An example of a reaction caused by irradiation with ultraviolet lighthaving a wavelength of lower than 200 nm and ultraviolet light having awavelength of 200 nm or higher in an atmosphere containing oxygen isshown below.O₂ +hν(λ₁ nm)→O(³P)+O(³P)  (1)O(³P)+O₂→O₃  (2)O₃ +hν(λ₂ nm)→O(¹D)+O₂  (3)

In the reaction formula (1), by irradiation with ultraviolet light (hν)having a wavelength of lower than 200 nm (λ₁ nm) in an atmospherecontaining oxygen (O₂), oxygen atoms in a ground state (O(³P)) aregenerated. Then, in the reaction formula (2), the oxygen atom in aground state (O(³P)) and oxygen (O₂) react with each other to generateozone (O₃). Next, in the reaction formula (3), irradiation withultraviolet light having a wavelength of 200 nm or higher (λ₂ nm) isperformed in an atmosphere containing the generated ozone (O₃), wherebysinglet oxygen in an excited state O(¹D) is generated. In an atmospherecontaining oxygen, ozone is generated by irradiation with ultravioletlight having a wavelength of lower than 200 nm, and the ozone isdecomposed to generate singlet oxygen by irradiation with ultravioletlight having a wavelength of 200 nm or higher. The above-described ozonetreatment can be performed by, for example, irradiation with alow-pressure mercury lamp (λ₁=185 nm, λ₂=254 nm) in an atmospherecontaining oxygen.

Further, an example of a reaction caused by irradiation with ultravioletlight having a wavelength of lower than 180 nm in an atmospherecontaining oxygen is shown below.O₂ +hν(λ₃ nm)→O(¹ D)+O(³P)  (4)O(³P)+O₂→O₃  (5)O₃ +hν(λ₃ nm)→O(¹ D)+O₂  (6)

In the reaction formula (4), by irradiation with ultraviolet lighthaving a wavelength of lower than 180 nm (λ₃ nm) in an atmospherecontaining oxygen (O₂), singlet oxygen in an excited state O(¹D) and anoxygen atom in a ground state (O(³P)) are generated. Then, in thereaction formula (5), the oxygen atom in a ground state (O(³P)) andoxygen (O₂) react with each other to generate ozone (O₃). In thereaction formula (6), irradiation with ultraviolet light having awavelength of lower than 180 nm (λ₃ nm) is performed in an atmospherecontaining the generated ozone (O₃), whereby singlet oxygen in anexcited state and oxygen are generated. By irradiation with ultravioletlight having a wavelength of lower than 180 nm in an atmospherecontaining oxygen, ozone is generated and the ozone or oxygen isdecomposed to generate singlet oxygen. The above-described ozonetreatment can be performed by, for example, irradiation with a Xeexcimer UV lamp (λ₃=172 nm) in an atmosphere containing oxygen.

Chemical bonds of organic substances or the like which are attached tothe surface of a processing object are cut by ultraviolet light having awavelength of lower than 200 nm Then, organic substances attached to thesurface of the processing object, organic substances whose chemicalbonds are cut, or the like can be oxidatively decomposed by ozone orsinglet oxygen generated from ozone to be removed. The above-describedozone treatment can increase the hydrophilic properties and cleanlinessof the surface of the processing object; accordingly, bonding can befavorably performed.

By irradiation with ultraviolet light in an atmosphere containingoxygen, ozone is generated. Ozone has an effect of removing organicsubstances attached to a surface of a processing object. In addition,singlet oxygen has an effect of removing organic substances attached toa surface of a processing object at a level equal to or more than theozone. Ozone and singlet oxygen are examples of oxygen in an activestate and also collectively referred to as active oxygen. As describedin the above reaction formulae and the like, there are a reaction inwhich ozone is generated in generation of singlet oxygen and a reactionin which singlet oxygen is generated from ozone. Therefore, reactions towhich singlet oxygen contributes are also referred to as ozone treatmentin convenience.

After the oxide film 132 and the nitrogen-containing layer 121 arebonded to each other, heat treatment is preferably performed in order toincrease the bonding strength. This heat treatment is performed at atemperature at which a crack is not generated in the embrittled region104 and is performed, for example, at a temperature higher than or equalto room temperature and lower than 400° C. The oxide film 132 and thenitrogen-containing layer 121 may be bonded to each other while beingheated at a temperature within the above range. This heat treatment canbe performed with a diffusion furnace, a heating furnace such as aresistance heating furnace, a rapid thermal annealing (RTA) apparatus, amicrowave heating apparatus, or the like.

In general, when heat treatment is performed at the same time as orafter bonding of the oxide film 132 and the nitrogen-containing layer121, a dehydration reaction at the bonding interface occurs and thebonding surfaces come closer to each other; thus, the bonding strengthis increased by strengthening of hydrogen bonding and formation ofcovalent bonding. In order to promote the dehydration reaction, moisturegenerated at the bonding interface through the dehydration reactionshould be removed by heat treatment at high temperature. In other words,when heat treatment after the bonding is performed at low temperature,moisture generated at the bonding interface through the dehydrationreaction cannot be removed effectively; thus, the dehydration reactiondoes not progress and it is difficult to increase the bonding strengthsufficiently.

On the other hand, in the case where an oxide film containing chlorineatoms or the like is used as the oxide film 132, the oxide film 132 canabsorb and diffuse moisture. Accordingly, even when the heat treatmentafter bonding is performed at low temperature, moisture generated at thebonding interface through the dehydration reaction can be absorbed anddiffused into the oxide film 132 and the dehydration reaction can bepromoted efficiently. In that case, even when a substrate having lowheat resistance such as a glass substrate is used as the base substrate120, the bonding strength between the oxide film 132 and thenitrogen-containing layer 121 can be substantially increased.Furthermore, when plasma treatment is performed by applying a biasvoltage, micropores as well as OH groups are formed in the vicinity ofthe surface of the oxide film 132, so that moisture can be effectivelyabsorbed and diffused into the oxide film 132. Accordingly, the bondingstrength between the oxide film 132 and the nitrogen-containing layer121 can be increased even when heat treatment is performed at lowtemperature.

Next, heat treatment is performed to cause separation along theembrittled region 104, whereby the single crystal semiconductor layer124 is provided over the base substrate 120 with the oxide film 132 andthe nitrogen-containing layer 121 interposed therebetween (see FIG. 2D).

When the heat treatment is performed, the element added is precipitatedinto crystal defects formed in the embrittled region 104 by temperatureincrease, and the internal pressure of the crystal defects increases.The increase in pressure changes the volume of the crystal defects inthe embrittled region 104 to generate cracks in the embrittled region104, whereby the single crystal semiconductor substrate 100 is separatedalong the embrittled region 104. Since the oxide film 132 is bonded tothe base substrate 120, the single crystal semiconductor layer 124 thatis separated from the single crystal semiconductor substrate 100 isprovided over the base substrate 120. This heat treatment is performedat a temperature that does not exceed the strain point of the basesubstrate 120, for example, at 400° C.

This heat treatment can be performed with a diffusion furnace, a heatingfurnace such as a resistance heating furnace, a rapid thermal annealing(RTA) apparatus, a microwave heating apparatus, or the like. Forexample, in the case of using a resistance heating furnace, heattreatment is performed at 200° C. for two hours and subsequentlyperformed at 600° C. for two hours. In the case of using an RTAapparatus, heat treatment can be performed at a temperature of 550° C.to 730° C. for a process time of 0.5 minutes to 60 minutes.

Note that instead of the aforementioned heat treatment for increasingthe bonding strength between the oxide film 132 and the base substrate120, heat treatment for increasing the bonding strength between theoxide film 132 and nitrogen-containing layer 121 and heat treatment forseparation along the embrittled region 104 may be performed at the sametime by performing the heat treatment of FIG. 1D. Specifically, heattreatment at 600° C. for two hours with a resistance heating furnace canincrease the bonding strength, repair defects in the single crystalsemiconductor substrate 100 which are caused by ion irradiation, anddesorb hydrogen.

Through the above steps, an SOI substrate in which the single crystalsemiconductor layer 124 is provided over the base substrate 120 with theoxide film 132 and the nitrogen-containing layer 121 interposedtherebetween can be manufactured. With the manufacturing methoddescribed in this embodiment, even in the case where thenitrogen-containing layer 121 is used as the bonding layer, the bondingstrength between the base substrate 120 and the single crystalsemiconductor layer 124 can be increased to increase reliability. Inaddition, the single crystal semiconductor layer 124 provided over thebase substrate 120 contains a predetermined concentration of nitrogen,oxygen, or carbon; thus, the single crystal semiconductor layer 124 canbe firmly bonded to the base substrate 120 and the mechanical strengthof the SOI substrate can be improved.

After that, as described in the above embodiment 1, the single crystalsemiconductor layer 124 is irradiated with a laser beam, whereby thecrystallinity of the single crystal semiconductor layer 124 can berecovered and the element such as nitrogen is solid-dissolved in oradded to the single crystal semiconductor layer 124. The laserirradiation step can be performed by utilizing the method and theapparatus described in the aforementioned embodiment. Note that theaddition of nitrogen or the like to the single crystal semiconductorlayer 124 may be performed in the step of adding hydrogen as describedin this embodiment, in the step of laser irradiation as described inEmbodiment 1, or in both of the steps.

Although the oxide film 132 is formed on the single crystalsemiconductor substrate 100 and the nitrogen-containing layer 121 isformed on the base substrate 120 in this embodiment, the presentinvention is not limited to that case. For example, the oxide film 132and the nitrogen-containing layer may be stacked over the single crystalsemiconductor substrate 100 in this order, and then the surface of thenitrogen-containing layer formed on the oxide film 132 may be bonded tothe surface of the base substrate 120. In that case, thenitrogen-containing layer may be provided before the formation of theembrittled region 104 or after the formation of the embrittled region104. Note that an oxide film (such as a silicon oxide film) may beformed on a nitrogen-containing layer and then the surface of the oxidefilm may be bonded to the surface of the base substrate 120.

In the case where intrusion of impurities such as sodium into the singlecrystal semiconductor layer 124 from the base substrate 120 does notcause any problems, the nitrogen-containing layer 121 does not need tobe provided on the base substrate 120, and the surface of the oxide film132 provided on the single crystal semiconductor substrate 100 may bebonded to the surface of the base substrate 120.

Note that the structure shown in this embodiment can be combined withstructures shown in other embodiments in this specification asappropriate.

(Embodiment 3)

In this embodiment, an apparatus for irradiating a single crystalsemiconductor layer provided over a base substrate with a laser beam (alaser irradiation apparatus) will be described with reference todrawings.

The laser irradiation apparatus described in this embodiment includes alaser oscillator 602 for emitting a laser beam 600 and a stage 606 overwhich a processing substrate 604 is arranged (see FIG. 3). A controller608 is connected to the laser oscillator 602. The energy, repetitionrate, and the like of the laser beam 600 emitted from the laseroscillator 602 can be changed by the controller 608. Furthermore, aheating means 610 such as a resistance heating device is provided in thestage 606 so as to heat the processing substrate 604.

The stage 606 is provided inside a chamber 612. The stage 606 isprovided to be able to move in the above chamber.

A part of the wall of the chamber 612 is provided with a window 616 forintroducing the laser beam 600 to the processing substrate 604. Thewindow 616 is formed of a material having a high transmittance withrespect to the laser beam 600, such as quartz. In order to increase thetransmittance with respect to the laser beam 600 as much as possible,the window 616 is preferably made thin.

In order to control the atmosphere in the chamber 612, a gas supply port620 and an exhaust port 622 are provided in the chamber 612. In thisembodiment, any one of nitrogen, oxygen, and carbon is contained in theatmosphere in the chamber 612. As a result, nitrogen, oxygen, or carbonis easily contained in the single crystal semiconductor layer in thelaser irradiation step.

The gas supply port 620 is provided with a gas supply device 628 througha valve. Although one gas supply device 628 is provided, a plurality ofgas supply devices may be provided to supply a plurality of gases intothe chamber 612 at the same time.

The exhaust port 622 is provided with an exhaust system. Specifically,the exhaust port 622 is provided with a turbo pump 632 and a dry pump634 through valves. In addition, the exhaust port 622 and the dry pump634 are connected to each other through the valves, and therefore, roughvacuum can be created by the dry pump.

An optical system 640 including a lens, a mirror, and the like isarranged between the laser oscillator 602 and the stage 606. The opticalsystem 640 is provided outside the chamber 612. The energy distributionof the laser beam 600 from the laser oscillator 602 is equalized by theoptical system 640, and the cross-sectional shape of the laser beam 600is processed into a linear shape or a rectangular shape. The laser beam600 which has passed through the optical system 640 passes through thewindow 618, enters inside the chamber 612, and is delivered on theprocessing substrate 604 over the stage 606. At this time, the entiresurface of the processing substrate 604 can be irradiated with the laserbeam 600 by moving the stage 606. Alternatively, the laser oscillator602 or the optical system 640 may be moved. Further alternatively, boththe processing substrate 604 and one of the laser oscillator 602 and theoptical system 640 may be moved. In the latter case, the processingsubstrate 604 and one of the laser oscillator 602 and the optical system640 are moved in different directions (for example, one is moved in theX-axis direction and the other is moved in the Y-axis direction that isrotated from the X-axis by 90°, whereby laser irradiation can beperformed efficiently.

When the processing substrate 604 is irradiated with the laser beam 600,the processing substrate 604 may be heated by the heating device in thestage 606 at a temperature equal to or lower than the strain point ofglass. Furthermore, when a nitrogen gas, an oxygen gas, and a gascontaining carbon as one of its components is selected as appropriate tobe supplied from the gas supply port 620, the laser beam 600 can beemitted in an atmosphere containing such a gas.

In this embodiment, the concentration of nitrogen, oxygen, or carbon inthe laser irradiation atmosphere is controlled by using the chamber. Byemploying such a method described in this embodiment, for example, theminimum concentration of nitrogen in the single crystal semiconductorlayer provided over the base substrate that is the processing substrate604 can be controlled to 5×10¹⁵ atoms/cm³ or more, preferably 1×10¹⁶atoms/cm³ or more, and more preferably 1×10¹⁷ atoms/cm³ or more. Theminimum concentration of oxygen in the single crystal semiconductorlayer can be controlled to 2×10¹⁸ atoms/cm³ or more, preferably 3×10¹⁸atoms/cm³ or more, and more preferably 5×10¹⁸ atoms/cm³ or more.Alternatively, the single crystal semiconductor layer may include aregion having an oxygen concentration of 2×10¹⁸ atoms/cm³ or more, andpreferably 5×10¹⁸ atoms/cm³ or more. The minimum concentration of carbonin the single crystal semiconductor layer can be controlled to 1×10¹⁷atoms/cm³ or more, and preferably 5×10¹⁷ atoms/cm³ or more. As a result,voids in the single crystal semiconductor layer can be reduced toincrease the yield stress.

Note that a too high concentration of each element may adversely affectthe characteristics of a transistor using such a single crystalsemiconductor layer. Therefore, it is preferable to control theconcentration of each element in the single crystal semiconductor layerso that the upper limit of the concentration of nitrogen is 5×10¹⁹atoms/cm³ or less, preferably 2×10¹⁹ atoms/cm³ or less, and morepreferably 5×10¹⁸ atoms/cm³ or less, the upper limit of theconcentration of oxygen is 1×10²⁰ atoms/cm³ or less, and preferably1×10¹⁹ atoms/cm³ or less, and the upper limit of the concentration ofcarbon is 5×10²⁰ atoms/cm³ or less, and more 5×10¹⁹ atoms/cm³ or less.

As the processing substrate 604 in this embodiment, an insulating filmcontaining at least one of nitrogen, oxygen, and carbon may be furtherprovided on the single crystal semiconductor layer provided over thebase substrate, and laser beam irradiation may be performed from asurface side of the insulating film. As the insulating film, forexample, a silicon nitride film, a silicon oxide film, a silicon nitrideoxide film (nitrogen>oxygen), or a silicon oxynitride film(oxygen>nitrogen) can be used. Such an insulating film may containcarbon at a concentration of about 1×10¹⁷ atoms/cm³ to 1×10²⁰ atoms/cm³.When the laser beam irradiation may be performed from a surface side ofthe insulating film, the concentration of nitrogen, oxygen, or carboncan be further increased in the single crystal semiconductor layer thatis re-single-crystallized. After the laser light irradiation, theinsulating film can be removed with hydrofluoric acid or bufferedhydrofluoric acid.

This embodiment shows an example in which the processing substrate 604is irradiated with the laser beam by using the chamber 612; however, thechamber is not necessarily used and the processing substrate may beirradiated with the laser beam in the atmosphere while being directlysprayed with a nitrogen gas, an oxygen gas, and a gas containing carbonas one of its components. For example, when the processing substrate issprayed with a nitrogen gas, both oxygen in the atmosphere and thenitrogen gas sprayed can be solid-dissolved in the single crystalsemiconductor layer.

Note that the structure shown in this embodiment can be combined withstructures shown in other embodiments in this specification asappropriate.

(Embodiment 4)

In this embodiment, a method for manufacturing a semiconductor deviceusing the SOI substrate manufactured in the above embodiment will bedescribed.

First, a method for manufacturing an n-channel thin film transistor anda p-channel thin film transistor will be described with reference toFIGS. 4A to 4D and FIGS. 5A to 5C. Various kinds of semiconductordevices can be formed by combining a plurality of thin film transistors(TFTs).

In this embodiment, description is made on the case where the SOIsubstrate manufactured through the steps of FIGS. 2A to 2D is used as anSOI substrate. Needless to say, an SOI substrate which is manufacturedusing any other method described in the above embodiments can be used.

FIG. 4A is a cross-sectional view of the SOI substrate manufactured bythe method described with reference to FIGS. 2A to 2D.

First, the singe crystal semiconductor layer 124 is divided for eachelement by etching to form semiconductor layers 251 and 252 asillustrated in FIG. 4B. The semiconductor layer 251 is included in ann-channel TFT, and the semiconductor layer 252 is included in ap-channel TFT.

As illustrated in FIG. 4C, an insulating film 254 is formed over thesemiconductor layers 251 and 252. Then, a gate electrode 255 is formedover the semiconductor layer 251 with the insulating film 254 interposedtherebetween, and a gate electrode 256 is formed over the semiconductorlayer 252 with the insulating film 254 interposed therebetween.

Before the single crystal semiconductor layer 124 is etched, an impurityelement serving as an acceptor, such as boron, aluminum, or gallium, oran impurity element serving as a donor, such as phosphorus or arsenic,is preferably added to the single crystal semiconductor layer 124 inorder to control the threshold voltage of the TFTs. For example, animpurity element serving as an acceptor is added to a region in which ann-channel TFT is to be formed, and an impurity element serving as adonor is added to a region in which a p-channel TFT is to be formed.

Next, as illustrated in FIG. 4D, n-type low-concentration impurityregions 257 are formed in the semiconductor layer 251, and p-typehigh-concentration impurity regions 259 are formed in the semiconductorlayer 252. Specifically, first, the n-type low-concentration impurityregions 257 are formed in the semiconductor layer 251. In order to formthe n-type low-concentration impurity regions 257, the semiconductorlayer 252 where the p-channel TFT is formed is covered with a resistmask, and an impurity element is added to the semiconductor layer 251.As the impurity element, phosphorus or arsenic may be added. By addingthe impurity element by an ion doping method or an ion implantationmethod, the gate electrode 255 functions as a mask, and the n-typelow-concentration impurity regions 257 are formed in the semiconductorlayer 251 in a self-aligned manner. A region of the semiconductor layer251 that overlaps the gate electrode 255 serves as a channel formationregion 258.

Next, after the mask that covers the semiconductor layer 252 is removed,the semiconductor layer 251 where the n-channel TFT is formed is coveredwith a resist mask. Then, an impurity element is added to thesemiconductor layer 252 by an ion doping method or an ion implantationmethod. As the impurity element, boron may be added. In the step ofadding the impurity element, the gate electrode 256 functions as a maskand the p-type high-concentration impurity regions 259 are formed in thesemiconductor layer 252 in a self-aligned manner. The p-typehigh-concentration impurity regions 259 serve as a source region or adrain region. A region of the semiconductor layer 252 that overlaps thegate electrode 256 serves as a channel formation region 260. Here,description is made on the method in which the p-type high-concentrationimpurity regions 259 are formed after the n-type low-concentrationimpurity regions 257 are formed; however, the p-type high-concentrationimpurity regions 259 can be formed first.

Next, after the resist that covers the semiconductor layer 251 isremoved, an insulating film having a single-layer structure or astacked-layer structure of a nitrogen compound such as silicon nitrideor an oxide such as silicon oxide is formed by plasma CVD or the like.This insulating film is anisotropically etched in a perpendiculardirection to form sidewall insulating films 261 and 262 that are incontact with side surfaces of the gate electrodes 255 and 256,respectively, as illustrated in FIG. 5A. By this anisotropic etching,the insulating film 254 is also etched.

Next, as illustrated in FIG. 5B, the semiconductor layer 252 is coveredwith a resist 265. In order to form high-concentration impurity regionsserving as a source region or a drain region in the semiconductor layer251, an impurity element is added to the semiconductor layer 251 at highdose by an ion implantation method or an ion doping method. The gateelectrode 255 and the sidewall insulating films 261 function as masks,and n-type high-concentration impurity regions 267 are formed. Then,heat treatment is performed to activate the impurity element.

After the heat treatment for activation, an insulating film 268containing hydrogen is formed as illustrated in FIG. 5C. After theinsulating film 268 is formed, heat treatment is performed at atemperature of 350° C. to 450° C., so as to diffuse hydrogen containedin the insulating film 268 into the semiconductor layers 251 and 252.The insulating film 268 can be formed by deposition of silicon nitrideor silicon nitride oxide by plasma CVD at a process temperature of 350°C. or less. The supply of hydrogen to the semiconductor layers 251 and252 makes it possible to efficiently correct defects that are to betrapping centers in the semiconductor layers 251 and 252 and at aninterface with the insulating film 254.

After that, an interlayer insulating film 269 is formed. The interlayerinsulating film 269 can have a single-layer structure or a stacked-layerstructure of any of films selected from an insulating film containing aninorganic material, such as a silicon oxide film or a BPSG(borophosphosilicate glass) film, and an organic resin film containingpolyimide, acrylic, or the like. After contact holes are formed in theinterlayer insulating film 269, wirings 270 are formed as illustrated inFIG. 5C. The wirings 270 can be formed of, for example, a conductivefilm having a three-layer structure in which a low-resistance metal filmsuch as an aluminum film or an aluminum-alloy film is sandwiched betweenbarrier metal films. The barrier metal films can be formed ofmolybdenum, chromium, titanium, or the like.

Through the aforementioned steps, a semiconductor device having then-channel TFT and the p-channel TFT can be manufactured. Since theconcentration of oxygen contained in the semiconductor layer in whichthe channel formation region is formed is reduced in the manufacturingprocess of the SOI substrate, a TFT with a low off current and lessvariations in threshold voltage can be manufactured.

Although the method for manufacturing TFTs is described with referenceto FIGS. 4A to 4D and FIGS. 5A to 5C, a semiconductor device with highadded value can be manufactured by forming various kinds ofsemiconductor elements such as a capacitor or a resistor as well as aTFT. Specific modes of semiconductor devices are described below withreference to drawings.

First, as an example of the semiconductor device, a microprocessor isdescribed. FIG. 6 is a block diagram illustrating a structural exampleof a microprocessor 500.

The microprocessor 500 includes an arithmetic logic unit (ALU) 501, anALU controller 502, an instruction decoder 503, an interrupt controller504, a timing controller 505, a register 506, a register controller 507,a bus interface (Bus I/F) 508, a read only memory (ROM) 509, and amemory interface 510.

Instructions input to the microprocessor 500 via the bus interface 508are input to the instruction decoder 503, decoded therein, and theninput to the ALU controller 502, the interrupt controller 504, theregister controller 507, and the timing controller 505. The ALUcontroller 502, the interrupt controller 504, the register controller507, and the timing controller 505 conduct various controls based on thedecoded instructions.

The ALU controller 502 generates signals for controlling the operationof the ALU 501. While the microprocessor 500 is executing a program, theinterrupt controller 504 judges and processes an interrupt request froman external input/output device or a peripheral circuit based on itspriority or a mask state. The register controller 507 generates anaddress of the register 506, and reads/writes data from/to the register506 in accordance with the state of the microprocessor 500. The timingcontroller 505 generates signals for controlling the timing of operationof the ALU 501, the ALU controller 502, the instruction decoder 503, theinterrupt controller 504, and the register controller 507. For example,the timing controller 505 is provided with an internal clock generatorfor generating an internal clock signal CLK2 based on a reference clocksignal CLK1. As illustrated in FIG. 6, the internal clock signal CLK2 isinput to other circuits.

Next, an example of a semiconductor device having an arithmetic functionand a function of communicating data wirelessly will be described. FIG.7 is a block diagram illustrating a structural example of such asemiconductor device. The semiconductor device illustrated in FIG. 7 canbe referred to as a computer (hereinafter referred to as an RFCPU) thatoperates by transmitting/receiving signals to/from an external device bywireless communication.

As illustrated in FIG. 7, an RFCPU 511 includes an analog circuitportion 512 and a digital circuit portion 513. The analog circuitportion 512 includes a resonance circuit 514 having a resonantcapacitor, a rectifier circuit 515, a constant voltage circuit 516, areset circuit 517, an oscillator circuit 518, a demodulation circuit519, and a modulation circuit 520. The digital circuit portion 513includes an RF interface 521, a control register 522, a clock controller523, an interface 524, a central processing unit 525, a random accessmemory 526, and a read only memory 527.

The operation of the RFCPU 511 is roughly described below. The resonancecircuit 514 generates induced electromotive force based on a signalreceived by an antenna 528. The induced electromotive force is stored ina capacitor portion 529 via the rectifier circuit 515. The capacitorportion 529 preferably includes a capacitor such as a ceramic capacitoror an electric double layer capacitor. The capacitor portion 529 is notnecessarily integrated over the same substrate as the RFCPU 511 and maybe incorporated into the RFCPU 511 as another component.

The reset circuit 517 generates a signal that resets and initializes thedigital circuit portion 513. For example, a signal that rises after anincrease in power supply voltage is generated as the reset signal. Theoscillator circuit 518 changes the frequency and duty ratio of a clocksignal in accordance with a control signal generated by the constantvoltage circuit 516. The demodulation circuit 519 demodulates a receivedsignal, and the modulation circuit 520 modulates data to be transmitted.

For example, the demodulation circuit 519 includes a low-pass filter andbinarizes a received signal of an amplitude shift keying (ASK) systembased on the variation of the amplitude. The modulation circuit 520transmits data by changing the amplitude of a transmission signal of theamplitude shift keying (ASK) system. Thus, the modulation circuit 520changes the resonance point of the resonance circuit 514, therebyvarying the amplitude of a communication signal.

The clock controller 523 generates a control signal for changing thefrequency and duty ratio of the clock signal in accordance with thepower supply voltage or the current consumption in the centralprocessing unit 525. The power supply voltage is monitored by a powersupply control circuit 530.

A signal that is input to the RFCPU 511 from the antenna 528 isdemodulated by the demodulation circuit 519, and then divided into acontrol command, data, and the like by the RF interface 521. The controlcommand is stored in the control register 522. The control commandincludes reading of data stored in the read only memory 527, writing ofdata to the random access memory 526, an arithmetic instruction to thecentral processing unit 525, and the like.

The central processing unit 525 accesses the read only memory 527, therandom access memory 526, and the control register 522 via the interface524. The interface 524 has a function of generating an access signal toany one of the read only memory 527, the random access memory 526, andthe control register 522 based on an address requested by the centralprocessing unit 525.

As an arithmetic method of the central processing unit 525, a method maybe employed in which an OS (operating system) is stored in the read onlymemory 527 and a program is read and executed at the time of startingoperation. Alternatively, a method may also be employed in which acircuit dedicated to arithmetic is formed and an arithmetic process isconducted using hardware. In a method using both hardware and software,part of arithmetic process can be conducted by a circuit dedicated toarithmetic, and the other part of the arithmetic process can beconducted by the central processing unit 525 using a program.

Next, display devices will be described with reference to FIGS. 8A and8B, and FIGS. 9A and 9B.

FIGS. 8A and 8B are drawings for describing a liquid crystal displaydevice. FIG. 8A is a plan view of a pixel of the liquid crystal displaydevice, and FIG. 8B is a cross-sectional view taken along line J-K ofFIG. 8A.

As illustrated in FIG. 8A, a pixel includes a single crystalsemiconductor layer 320, a scanning line 322 intersecting with thesingle crystal semiconductor layer 320, a signal line 323 intersectingwith the scanning line 322, a pixel electrode 324, and an electrode 328that electrically connects the pixel electrode 324 to the single crystalsemiconductor layer 320. The single crystal semiconductor layer 320 is alayer formed using the single crystal semiconductor layer provided overthe base substrate 120 and is included in a TFT 325 of the pixel.

As an SOI substrate, the SOI substrate described in the aboveembodiments is used. As illustrated in FIG. 8B, the single crystalsemiconductor layer 320 is stacked over the base substrate 120 with theoxide film 132 and the nitrogen-containing layer 121 interposedtherebetween. A glass substrate can be used as the base substrate 120.The single crystal semiconductor layer 320 of the TFT 325 is a film thatis obtained by etching the single crystal semiconductor layer of the SOIsubstrate. A channel formation region 340 and n-type high-concentrationimpurity regions 341 to which an impurity element is added are formed inthe single crystal semiconductor layer 320. A gate electrode of the TFT325 is included in the scanning line 322 and one of a source electrodeand a drain electrode of the TFT 325 is included in the signal line 323.

The signal line 323, the pixel electrode 324, and the electrode 328 areprovided over an interlayer insulating film 327. Columnar spacers 329are formed over the interlayer insulating film 327. An orientation film330 is formed to cover the signal line 323, the pixel electrode 324, theelectrode 328, and the columnar spacers 329. A counter substrate 332 isprovided with a counter electrode 333 and an orientation film 334 thatcovers the counter electrode 333. The columnar spacers 329 are formed tomaintain the space between the base substrate 120 and the countersubstrate 332. A liquid crystal layer 335 is formed in the space formedby the columnar spacers 329. At connection portions of the signal line323 and the electrode 328 with the high-concentration impurity regions341, there are steps formed in the interlayer insulating film 327 due toformation of contact holes; thus, liquid crystal orientation in theliquid crystal layer 335 at these connection portions is likely to bedisordered. Therefore, the columnar spacers 329 are formed at thesesteps to prevent the liquid crystal orientation from being disordered.

Next, an electroluminescence display device (hereinafter referred to asan EL display device) will be described with reference to FIGS. 9A and9B. FIG. 9A is a plan view of a pixel of the EL display device, and FIG.9B is a cross-sectional view taken along line J-K of FIG. 9A.

As illustrated in FIG. 9A, a pixel includes a TFT as a selectiontransistor 401, a TFT as a display control transistor 402, a scanningline 405, a signal line 406, a current supply line 407, and a pixelelectrode 408. In the EL display device, each pixel is provided with alight-emitting element having a structure in which a layer containing anelectroluminescent material (an EL layer) is sandwiched between a pairof electrodes. One electrode of the light-emitting element is the pixelelectrode 408. Furthermore, a semiconductor layer 403 includes a channelformation region, a source region, and a drain region of the selectiontransistor 401. A semiconductor layer 404 includes a channel formationregion, a source region, and a drain region of the display controltransistor 402. The semiconductor layers 403 and 404 are layers that areformed using the single crystal semiconductor layer 124 provided overthe base substrate.

In the selection transistor 401, a gate electrode is included in thescanning line 405, one of a source electrode and a drain electrode isincluded in the signal line 406, and the other thereof is formed as anelectrode 411. In the display control transistor 402, a gate electrode412 is electrically connected to the electrode 411, one of a sourceelectrode and a drain electrode is formed as an electrode 413 that iselectrically connected to the pixel electrode 408, and the other thereofis included in the current supply line 407.

The display control transistor 402 is a p-channel TFT. As illustrated inFIG. 9B, a channel formation region 451 and p-type high-concentrationimpurity regions 452 are formed in the semiconductor layer 404. Notethat as an SOI substrate, the SOI substrate manufactured in theaforementioned embodiments is used.

An interlayer insulating film 427 is formed to cover the gate electrode412 of the display control transistor 402. The signal line 406, thecurrent supply line 407, the electrode 411, the electrode 413, and thelike are formed over the interlayer insulating film 427. The pixelelectrode 408 that is electrically connected to the electrode 413 isformed over the interlayer insulating film 427. A peripheral portion ofthe pixel electrode 408 is surrounded by a partition wall layer 428having an insulating property. The EL layer 429 is formed over the pixelelectrode 408, and a counter electrode 430 is formed over the EL layer429. A counter substrate 431 is provided as a reinforcing plate andfixed to the base substrate 120 by a resin layer 432.

The gray scale of the EL display device can be controlled by a currentdriving method in which the luminance of a light-emitting element iscontrolled by current or a voltage driving method in which the luminanceof a light-emitting element is controlled by voltage. In the case wherethe transistors of different pixels have largely differentcharacteristic values, it is difficult to employ the current drivingmethod; in order to employ the current driving method in such a case, acorrection circuit for correcting characteristic variations is needed.When the EL display device is manufactured by a method includingmanufacturing steps of an SOI substrate, the selection transistor 401and the display control transistor 402 do not have variations incharacteristics in different pixels. Thus, the current driving methodcan be employed.

That is, a variety of electronic devices can be manufactured using theSOI substrate. The electronic devices include, in its category, camerassuch as video cameras and digital cameras, navigation systems, audioreproducing devices (such as car audio sets and audio components),computers, game machines, portable information terminals (such as mobilecomputers, cellular phones, portable game machines, and e-book readers),and image reproducing devices having recording media (specifically,devices provided with display devices capable of playing audio datastored in recording media such as a digital versatile disc (DVD) anddisplaying stored image data). An example of them is illustrated inFIGS. 10A to 10C.

FIGS. 10A to 10C illustrate an example of a cellular phone to which anembodiment of the invention disclosed in this specification is applied.FIG. 10A is a front view, FIG. 10B is a rear view, and FIG. 10C is afront view in which two housings are slid. A cellular phone 700 includestwo housings: a housing 701 and a housing 702. The cellular phone 700 isa so-called smartphone that has both functions of a cellular phone and aportable information terminal and incorporates a computer, and thus iscapable of a variety of data processing in addition to voice calls.

The cellular phone 700 includes the housing 701 and the housing 702. Thehousing 701 includes a display portion 703, a speaker 704, a microphone705, operation keys 706, a pointing device 707, a front camera lens 708,a jack 709 for an external connection terminal, an earphone terminal710, and the like. The housing 702 includes a keyboard 711, an externalmemory slot 712, a rear camera 713, a light 714, and the like. Inaddition, an antenna is incorporated in the housing 701.

Furthermore, in addition to the above structure, a wireless IC chip, asmall memory device, or the like may be incorporated in the cellularphone 700.

The housings 701 and 702 that overlap each other (see FIG. 10A) can beslid, and are developed by being slid as illustrated in FIG. 10C. Thedisplay panel or the display device that is manufactured by the methodsfor manufacturing a display device described in Embodiments 2 and 3 canbe incorporated in the display portion 703. Since the front camera lens708 is provided in the same plane as the display portion 703, thecellular phone 700 can be used as a videophone. Furthermore, by usingthe display portion 703 as a viewfinder, still images and moving imagescan be taken with the rear camera 713 and the light 714.

By using the speaker 704 and the microphone 705, the cellular phone 700can be used as an audio recording device (a recording device) or anaudio reproducing device. In addition, with the use of the operationkeys 706, it is possible to perform operations of incoming and outgoingof calls, simple information input such as e-mails, scrolling of ascreen to be displayed on the display portion, cursor movement, e.g.,for selecting information to be displayed on the display portion, andthe like.

If a large amount of information needs to be treated in documentation,in the use as a portable information terminal, and the like, it isconvenient to use the keyboard 711. By sliding the housings 701 and 702that overlap each other (FIG. 10A), the housings 701 and 702 can bedeveloped as illustrated in FIG. 10C. In using the cellular phone 700 asa portable information terminal, a cursor can be moved smoothly with theuse of the keyboard 711 and the pointing device 707. The jack 709 for anexternal connection terminal can be connected to an AC adapter or avariety of cables such as a USB cable, thereby performing charging anddata communication with a personal computer or the like. Furthermore, byinserting a recording medium into the external memory slot 712, a largeramount of data can be stored and moved.

The rear face of the housing 702 (FIG. 10B) is provided with the rearcamera 713 and the light 714, and still images and moving images can betaken using the display portion 703 as a viewfinder.

Furthermore, in addition to the above functions and structures, thecellular phone 700 may have an infrared communication function, a USBport, a function of receiving one segment television broadcast, awireless IC chip, an earphone jack, or the like.

The electronic device described with reference to FIGS. 10A to 10C canbe manufactured by any of the aforementioned methods for manufacturing atransistor and a display device.

Note that the structure shown in this embodiment can be combined withstructures shown in other embodiments in this specification asappropriate.

EXAMPLE 1

In this example, an example of the result of a measurement of theconcentration of nitrogen and oxygen in a single crystal semiconductorlayer that can be used in Embodiment 1 will be shown in FIGS. 11A and11B. Specifically, FIGS. 11A and 11B show the concentration distributionof nitrogen and oxygen in the depth direction of an SOI substrate inwhich a single crystal silicon layer is provided over a base substratewith a silicon oxide film interposed therebetween, in the case wherelaser irradiation is performed or not. FIG. 11A shows the concentrationdistribution of nitrogen and FIG. 11B shows the concentrationdistribution of oxygen.

The thickness of the single crystal silicon layer was set to about 130nm, and the measurement was performed by a SIMS apparatus (PHIADEPT-1010 manufactured by ULVAC-PHI, Incorporated). The measurement wasperformed with Cs⁺ as primary ions, an acceleration voltage of 5.0 kV, adetection region of 140 μm×140 μm, and a sputtering rate of 0.3 nm/sec,and electron-beam irradiation was performed to compensate the surfacecharge build-up.

The laser beam was emitted in the following manner.

A stage for supporting the base substrate, which was kept at roomtemperature, was sprayed with a nitrogen gas. At the same time, a samplewas irradiated with a XeCl excimer laser beam emitted from a laseroscillator (LAMBDA STEEL 670 manufactured by Lambda Physik Co., Ltd.),which had a repetition rate of 30 Hz, a pulse width of 22±5 nsec, anenergy density of 697 mJ/cm², and a wavelength of 308 nm The sample wasscanned at a rate of 1 mm/sec., and each part of the sample wasirradiated with the laser beam 10 times.

In a region of the single crystal silicon layer from the surface to adepth of about 10 nm, each concentration of oxygen and nitrogen measuredby SIMS can be detected to be higher due to surface roughness of thesingle crystal silicon layer, knock-on effect, or a gas componentremaining in the atmosphere, and thus an accurate concentration in thelayer cannot always be detected. Furthermore, in a region from a depthof 120 nm to a depth of 140 nm, the single crystal silicon layer cannotbe completely isolated from the silicon oxide film provided under thesingle crystal silicon layer, and the concentration of the silicon oxidefilm is also likely to be detected.

In a region from a depth of 20 nm to 70 nm in FIGS. 11A and 11B, asample that is not irradiated with a laser beam has a nitrogenconcentration of about 3×10¹⁶ atoms/cm³ to 3×10¹⁷ atoms/cm³, and anoxygen concentration of about 2×10¹⁸ atoms/cm³ to 1×10¹⁹ atoms/cm³. Onthe other hand, in the case where a sample is irradiated with a laserbeam while being sprayed with a nitrogen gas, a constant concentrationvalue is obtained: the concentration of nitrogen in the thicknessdirection is about 2×10¹⁷ atoms/cm³, and the concentration of oxygen inthe thickness direction is about 5×10¹⁸ atoms/cm³ to 1×10¹⁹ atoms/cm³.

When a high concentration of nitrogen and oxygen is contained in thedepth direction of the single crystal silicon layer, nitrogen atoms andoxygen atoms are trapped in the dislocation to fix the dislocation,whereby the yield stress of the entire single crystal silicon layer canbe increased. The atoms are trapped in the dislocation not only in astep of cooling after laser light irradiation but also in a heattreatment step in a manufacturing process of a device. The nitrogenatoms can also prevent a change in the shape of voids in a singlecrystal semiconductor layer and reduce the size of the voids. In FIGS.11A and 11B, the sample is sprayed with a nitrogen gas in theatmosphere; thus, oxygen in the atmosphere is contained in the singlecrystal silicon layer in addition to nitrogen. Accordingly, theconcentration of oxygen increases as shown in FIG. 11B.

As for a sample that is not irradiated with a laser beam, nitrogen andoxygen are added when a single crystal silicon substrate is doped withhydrogen to form an embrittled region. Thus, nitrogen with aconcentration of 1×10¹⁶ atoms/cm³ or more and oxygen with aconcentration of 2×10¹⁸ atoms/cm³ or more are contained in the singlecrystal silicon layer, and a sufficient mechanical strength can beobtained.

Although an example of using a single crystal silicon layer is shown inFIGS. 11A and 11B, a similar phenomenon can be observed even in the caseof using other single crystal semiconductor thin films. Thus, theconcentrations of nitrogen and oxygen in the single crystalsemiconductor layer can be equalized and increased by laser lightirradiation, which allows the crystallinity of the single crystalsemiconductor layer to be recovered and the mechanical strength to beimproved.

EXAMPLE 2

In Example 1, an example of directly irradiating the surface of a singlecrystal silicon layer with a laser beam is shown. In this example, FIG.12 shows an example of the result of a measurement of the concentrationof nitrogen in the case where a silicon oxynitride film is provided on asingle crystal semiconductor layer. Specifically, FIG. 12 shows theconcentration distribution of nitrogen in the depth direction of an SOIsubstrate in which a single crystal silicon layer is provided over abase substrate with a silicon oxide film interposed therebetween and asilicon oxynitride film is further provided on the single crystalsilicon layer, in the case where laser irradiation is performed or not.

The measurement was performed by a SIMS apparatus (PHI ADEPT-1010manufactured by ULVAC-PHI, Incorporated). The measurement was performedwith Cs⁺ as primary ions, an acceleration voltage of 5.0 kV, a detectionregion of 60 μm×77 μm, and a sputtering rate of 0.4 nm/sec., andelectron-beam irradiation was performed to compensate the surface chargebuild-up. The thickness of the single crystal silicon layer was set toabout 130 nm and the thickness of the silicon oxynitride film was set toabout 280 nm.

The laser light was emitted in the following manner.

A stage for supporting the base substrate, which was kept at roomtemperature, was sprayed with a nitrogen gas. At the same time, a samplewas irradiated with a XeCl excimer laser beam emitted from a laseroscillator (LAMBDA STEEL 670 manufactured by Lambda Physik Co., Ltd.,),which had a repetition rate of 30 Hz, a pulse width of 22±5 nsec, anenergy density of 689.4 mJ/cm², and a wavelength of 308 nm Each part ofthe sample was irradiated with the laser beam 10 times.

In a sample that was not irradiated with a laser beam, the concentrationof nitrogen in the single crystal silicon layer was 1×10¹⁶ atoms/cm³ ormore. In a sample that was irradiated with a laser beam, theconcentration of nitrogen in the single crystal silicon layer was 1×10¹⁸atoms/cm³ or more. By irradiating the single crystal silicon layer witha laser beam after the formation of the silicon oxynitride film,nitrogen in the silicon oxynitride film is solid-dissolved in or addedto the single crystal silicon layer when the surface of the singlecrystal silicon layer is melted. Accordingly, the single crystal siliconlayer can contain nitrogen at a high concentration of 1×10¹⁸ atoms/cm³or more. Note that also in the sample that is not irradiated with alaser beam, nitrogen is added when a single crystal silicon substrate isdoped with hydrogen to form an embrittled region; thus, nitrogen with aconcentration of 1×10¹⁶ atoms/cm³ or more is contained in the singlecrystal silicon layer, and a sufficient mechanical strength can beobtained.

Although an example of using a single crystal silicon layer is shown inFIG. 12, a similar phenomenon can be observed even in the case of usingother single crystal semiconductor thin films. Thus, the concentrationof nitrogen in the single crystal semiconductor layer can be equalizedand increased by laser light irradiation, which allows the crystallinityof the single crystal semiconductor layer to be recovered and themechanical strength to be improved.

EXAMPLE 3

This example shows the case where nitrogen, oxygen, and carbon as wellas hydrogen are added to a single crystal silicon substrate including asilicon oxide film formed by thermal oxidation. FIGS. 13A and 13B showthe result of a measurement of the concentrations of hydrogen, nitrogen,oxygen, and carbon in the single crystal silicon substrate.

The measurement of hydrogen in FIGS. 13A and 13B was performed by a SIMSapparatus (Physical Electronics PHI 6650 manufactured by ULVAC-PHI,Incorporated). The measurement was performed with Cs⁺ as primary ions,an acceleration voltage of 5.0 kV, a detection region of 60 μm×77 μm,and a sputtering rate of 0.9 nm/sec., and electron-beam irradiation wasperformed to compensate the surface charge build-up.

The measurement of nitrogen, oxygen, and carbon was performed by a SIMSapparatus (PHI ADEPT 1010 manufactured by ULVAC-PHI, Incorporated). Themeasurement was performed with Cs⁺ as primary ions, an accelerationvoltage of 5.0 kV, a detection region of 90 μm×90 μm, and a sputteringrate of 0.5 nm/sec., and electron-beam irradiation was performed tocompensate the surface charge build-up. The thickness of the siliconoxide film on the single crystal silicon substrate was set to about 100nm. It is noted that in FIGS. 13A and 13B, the measurement was performedfrom the back surface of the single crystal silicon substrate in orderto prevent effects of detecting components in silicon oxide on thesingle crystal silicon substrate as much as possible. After the singlecrystal silicon substrate was polished to a predetermined thickness, themeasurement was performed from the back surface of the single crystalsilicon substrate and finished beyond the boundary between the siliconoxide film and the single crystal silicon substrate. Therefore, siliconoxide is present in a region from a depth of 0 nm to a depth of 40 nm inFIGS. 13A and 13B.

Doping conditions are different between FIG. 13A and FIG. 13B. In FIG.13A, hydrogen was introduced at a flow rate of 24 sccm, and ions wereextracted from plasma generated by RF discharge and accelerated at anacceleration voltage of 40 kV. In FIG. 13B, hydrogen was introduced at aflow rate of 50 sccm, and ions were extracted from plasma generated byfilament discharge and accelerated at an acceleration voltage of 50 kV.

FIG. 13A shows that nitrogen, oxygen, and carbon are added with a gentleconcentration gradient in the depth direction of the single crystalsilicon substrate from the boundary between the silicon oxide film andthe single crystal silicon substrate to a depth of about 120 nm Thehydrogen concentration has a peak at about 150 nm, and a highconcentration of nitrogen, oxygen, and carbon was able to be added in aregion that is separated from the single crystal silicon substrate laterto be a single crystal silicon layer.

FIG. 13B shows that nitrogen, oxygen, and carbon are added with a gentleconcentration gradient in the depth direction of the single crystalsilicon substrate from the boundary between the silicon oxide film andthe single crystal silicon substrate to a depth of about 180 nm. Thehydrogen concentration has a peak at about 150 nm, which is the same asin FIG. 13A, and nitrogen, oxygen, and carbon was able to be added at ahigher concentration than in the case of FIG. 13B in a region that isseparated from the single crystal silicon substrate later to be a singlecrystal silicon layer.

This application is based on Japanese Patent Application serial No.2008-271676 filed with Japan Patent Office on Oct. 22, 2008, the entirecontents of which are hereby incorporated by reference.

1. A method for manufacturing an SOI substrate, comprising the steps of:adding hydrogen to a single crystal semiconductor substrate so that anembrittled region is formed in a region at a predetermined depth from asurface of the single crystal semiconductor substrate; bonding thesingle crystal semiconductor substrate to a base substrate with aninsulating layer interposed therebetween; heating the single crystalsemiconductor substrate to be separated along the embrittled region,whereby a semiconductor layer is provided over the base substrate withthe insulating layer interposed therebetween; and melting at least asuperficial part of the semiconductor layer, wherein oxygen is added tothe semiconductor layer in the melting step so that the semiconductorlayer includes a region having an oxygen concentration of 2×10¹⁸atoms/cm³ or more.
 2. The method for manufacturing an SOI substrate,according to claim 1, wherein nitrogen is added to the semiconductorlayer when the semiconductor layer is melted, so that a concentration ofnitrogen in the semiconductor layer is 5×10¹⁵ atoms/cm³ or more.
 3. Themethod for manufacturing an SOI substrate according to claim 1, whereincarbon is further added to the semiconductor layer in the melting step,so that a concentration of carbon in the semiconductor layer is 1×10¹⁷atoms/cm³ or more.
 4. The method for manufacturing an SOI substrateaccording to claim 1, wherein melting is performed by irradiating thesemiconductor layer with a laser beam.
 5. The method for manufacturingan SOI substrate according to claim 1, wherein melting is performed bystrong light irradiation with a lamp.
 6. The method for manufacturing anSOI substrate according to claim 1, wherein the superficial part of thesemiconductor layer is melted while being sprayed with a gas containingat least one of nitrogen, oxygen, and carbon.
 7. The method formanufacturing an SOI substrate according to claim 1, wherein at leastone of nitrogen, oxygen, and carbon is added to the single crystalsemiconductor substrate at the same time as the addition of hydrogen. 8.The method for manufacturing an SOI substrate according to claim 1,wherein a silicon nitride film, a silicon oxide film, a siliconoxynitride film, or a silicon nitride oxide film is formed over thesemiconductor layer before the superficial part of the semiconductorlayer is melted.
 9. The method for manufacturing an SOI substrateaccording to claim 1, wherein a glass substrate is used as the basesubstrate.
 10. A method for manufacturing a semiconductor devicecomprising a transistor which is formed using the SOI substrateaccording to claim
 1. 11. A method for manufacturing an SOI substrate,comprising the steps of: adding hydrogen to a single crystalsemiconductor substrate so that an embrittled region is formed in aregion at a predetermined depth from a surface of the single crystalsemiconductor substrate; bonding the single crystal semiconductorsubstrate to a base substrate with an insulating layer interposedtherebetween; heating the single crystal semiconductor substrate to beseparated along the embrittled region, whereby a semiconductor layer isprovided over the base substrate with the insulating layer interposedtherebetween; and melting at least a superficial part of thesemiconductor layer, wherein nitrogen is added to the single crystalsemiconductor substrate at the same time as the addition of hydrogen andto the semiconductor layer in the melting step, so that thesemiconductor layer includes a region having a nitrogen concentration of1×10¹⁶ atoms/cm³ or more, and wherein oxygen is added to the singlecrystal semiconductor substrate at the same time as the addition ofhydrogen and to the semiconductor layer in the melting step, so that thesemiconductor layer includes a region having an oxygen concentration of2×10¹⁸ atoms/cm³ or more.
 12. The method for manufacturing an SOIsubstrate, according to claim 11, wherein carbon is added to the singlecrystal semiconductor substrate at the same time as the addition ofhydrogen, so that a concentration of carbon in the semiconductor layeris 1×10¹⁷ atoms/cm³ or more.
 13. The method for manufacturing an SOIsubstrate, according to claim 11, wherein a glass substrate is used asthe base substrate.
 14. A method for manufacturing a semiconductordevice comprising a transistor which is formed using the SOI substrateaccording to claim
 11. 15. The method for manufacturing an SOI substrateaccording to claim 11, wherein melting is performed by irradiating thesemiconductor layer with a laser beam.
 16. The method for manufacturingan SOI substrate according to claim 11, wherein melting is performed bystrong light irradiation with a lamp.
 17. The method for manufacturingan SOI substrate according to claim 11, wherein the superficial part ofthe semiconductor layer is melted while being sprayed with a gascontaining at least one of nitrogen, oxygen, and carbon.
 18. The methodfor manufacturing an SOI substrate according to claim 11, wherein asilicon nitride film, a silicon oxide film, a silicon oxynitride film,or a silicon nitride oxide film is formed over the semiconductor layerbefore the superficial part of the semiconductor layer is melted.